Contrary to a widely held teaching, a person can be forgiven if they repent of receiving the mark and worshipping the image of the beast, just as they can be forgiven of any other idolatry that they repent of. Idolatry is not blasphemy of the Holy Spirit so according to the words of Jesus, it will be forgiven if repented of.
“Therefore I say to you, any sin and blasphemy shall be forgiven people, but blasphemy against the Spirit shall not be forgiven." Matthew 12:31
That fact in itself should cause those who claim that receiving the mark is unforgivable to reexamine why they think the way they do. In teaching that the mark is unforgivable, they are essentially teaching something contrary to what Jesus said, and it goes against the whole message of the new covenant which is essentially repentance for the forgiveness of sins.

Receiving the mark and worshipping the image of the beast is a serious sin. Those who persist in it come under God's judgment. They are the ones affected by the seven plagues and will continue to be so until they quit receiving of the benefits of the mark and quit worshipping the image. They have to utterly and completely forsake the false prophet regardless of the consequences.

The plagues are meant to convince the people of the world that they were deceived into believing that the false prophet is God when in fact, he's only a deceiver. God will send two witnesses to confront the false prophet and declare to the world the truth of what's taking place. They will have a role similar to that of Aaron and Moses when they confronted Pharaoh and sent the plagues on Egypt. The first four plagues will affect the fresh water, the seas, the sun, moon, and stars in order to demonstrate that the false prophet is no God at all, that he is powerless against the true God, and that anyone worshipping him should repent.

There will come a time during the plagues when people’s hearts will harden beyond repentance but that point isn’t reached until the end of the fourth plague. Until then, God is demonstrating His power over His creation in contrast to the false prophet’s impotence in hopes that people will see through his deception and repent. When Jesus returns at the seventh plague, all decisions are final.