Forgiving 70 times 7 is for us now. So we can get along with one another. We can't put someone into the eternal kingdom by forgiving them over and over again. Only Jesus can. If we could do such a thing, then we might as well start praying for the dead and get them into heaven as well.
Just because the scripture says that we cover a multitude of sins when we forgive someone is not talking about salvation... just creating unity among us.
A person would have to fall away completely from God and never look back to be lost forever. God can forgive anyone if they are repentant and sincere about it.