And that is exactly what the passage says.
And the reason they can't be brought back to repentance and saved is, not because it's impossible to fall away (the passage says nothing about that), but because "they are crucifying the Son of God all over again and subjecting him to public disgrace" - Hebrews 6:4-6.
You see, God is preventing them from coming back (for the reason stated), not preventing them from falling away. The thing that is impossible is for the fallen person to come back to Christ. The passage plainly says that. But I suppose if one is bent on defending a certain belief it can't read it for what it actually says.
Its like I was predicting the future...
Of course, if you were trying really hard to prove that once a person was flying they could stop flying mid-flight then you would ignore this aspect of the impossible situation. You would just say, "see right there, it says you can stop flying mid flight".
There sure are a LOT of passages you MUST ignore in order to insert your (wrong) suppositions into Hebrew 6:4-6
But that's kind of the whole problem with legalists and judaizers. They ignore the promises of God and insert their wrong suppositions in order to support their silliness.
John 10:27-28
27 My sheep hear my voice, and I know them, and they follow me:
28 And I give unto them eternal life; and they shall never perish,
neither shall any man pluck them out of my hand.
What you are trying to assert in Hebrews 6 is a direct contradiction of the Lord Jesus Christ.
You are trying to make an impossible hypothetical situation into something that not only can happen but DOES. Even though the Lord Jesus has said it cannot happen.
Hebrews 13:5 Let your conversation be without covetousness; and be content with such things as ye have: for he hath said,
I will never leave thee, nor forsake thee.
Let that one marinate for a bit.