It's best summed up in these verses:
Heb 6:4 For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit,
Heb 6:5 and have tasted the good word of God and the powers of the age to come,
Heb 6:6 if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame.
The unpardonable sin is rejecting the salvation one has through the holy spirit of the Lord.
Luk 12:10 "And anyone who speaks a word against the Son of Man, it will be forgiven him; but to him who blasphemes against the Holy Spirit, it will not be forgiven.
Anyone who has gone so far as to reject the salvation of the Lord once it has been given will get to the point where they blaspheme against the same life giving spirit.
So would a person who once was a member and has left the church have committed the unpardonable sin? Would they be disfellowshipped and/or marked?