It is true that those that practise fornications will be called fornicators as those that commit adultery will be called adulterers, so then those that practise wickedness shall be known by their wickedness, otherwise how can sinners say they have repented from their sins in the sight and ears of others? One cannot cease to be a hypocrit unless declared what the hypocrisey is to stop being double-minded about.
"Stiff neck" usually refers to the religious pride of the person that is exalting himself or herself over others, and are bent on continuing out of rebellion, but I have not see God calling anyone a stiff necked.
Deuteronomy 31:27For I know thy rebellion, and thy stiff neck: behold, while I am yet alive with you this day, ye have been rebellious against the LORD; and how much more after my death?
Moses said that above in verses 25-27. The Lord wasn't going to die.
2 Chronicles 36:13And he also rebelled against king Nebuchadnezzar, who had made him swear by God: but he stiffened his neck, and hardened his heart from turning unto the LORD God of Israel.
This is referring to what the rebellious did.
Psalm 75:5Lift not up your horn on high: speak not with a stiff neck.
This is a warning to not become prideful in exalting yourself over others.
Jeremiah 17:23But they obeyed not, neither inclined their ear, but made their neck stiff, that they might not hear, nor receive instruction.
Again, this is referring to what the rebellious were doing.
"Hard headed" term was not found in the King James Bible search engine at Bible Gateway.
The point of the OP is that "getting through a person's thick skull" is not your job for God causes the increase, and by trying to get through a believer's thick skull is tempting one to sin by looking down on him as if he is stupid but by doing so, void faith in Him as He is the One doing the work and thereby voiding that love to correct others by belittling them as if God is unable to save or to correct that person that has been bought as His.
Even broad based ethnic prejudices are frowned upon as if God would not deal with them.
Titus 1:10For there are many unruly and vain talkers and deceivers, specially they of the circumcision: 11Whose mouths must be stopped, who subvert whole houses, teaching things which they ought not, for filthy lucre's sake. 12One of themselves, even a prophet of their own, said, the Cretians are alway liars, evil beasts, slow bellies. 13This witness is true. Wherefore rebuke them sharply, that they may be sound in the faith;
We are called to love one another,
1 John 4:19We love him, because he first loved us. 20If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen? 21And this commandment have we from him, That he who loveth God love his brother also.
...and that is how we are known as His by others.
1 John 3:10In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother. 11For this is the message that ye heard from the beginning, that we should love one another.
Have you ever heard a name calling done in love?
And yet how we live should reflect by His grace in how we minister.
Galatians 3:14That the blessing of Abraham might come on the Gentiles through Jesus Christ; that we might receive the promise of the Spirit through faith..........26For ye are all the children of God by faith in Christ Jesus.
Hebrews 12:1Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, 2Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God. 3For consider him that endured such contradiction of sinners against himself, lest ye be wearied and faint in your minds. 4Ye have not yet resisted unto blood, striving against sin. 5And ye have forgotten the exhortation which speaketh unto you as unto children, My son, despise not thou the chastening of the Lord, nor faint when thou art rebuked of him: 6For whom the Lord loveth he chasteneth, and scourgeth every son whom he receiveth. 7If ye endure chastening, God dealeth with you as with sons; for what son is he whom the father chasteneth not? 8But if ye be without chastisement, whereof all are partakers, then are ye bastards, and not sons. 9Furthermore we have had fathers of our flesh which corrected us, and we gave them reverence: shall we not much rather be in subjection unto the Father of spirits, and live? 10For they verily for a few days chastened us after their own pleasure; but he for our profit, that we might be partakers of his holiness. 11Now no chastening for the present seemeth to be joyous, but grievous: nevertheless afterward it yieldeth the peaceable fruit of righteousness unto them which are exercised thereby.