Of course what is interesting is that in verse 6, it does not say 'someone' its says 'that which' or 'what'. the Greek is neuter .
So what is holding back the lawless one? This will help you understand what needs to be taken out of the way.
Someone indeed, 'as it is written' according to verse 7;
"........................................................ Only he who now restrains it will do so until he is out of the way."
GOD bless you in the name of LORD JESUS CHRIST.