Jesus said not to swear to God no matter what as in he does not seem to care for it, so it does not sound like something he would consider some sacred breach. Really the sin would be using the lord's name in vain. In vain means without success or a result.
Now if someone is saved and they have the seal of God on their forehead and their name is written in the book of life, that means according to the bible they are going to heaven. And there is not some little incantation someone says that undoes that. Unless they really want it undone and do not want to be saved. However something like that can anger God and who that believes in God would want him angry with them?
Jesus said there is an unpardonable sin called the blasphemy of the Holy Spirit, but I think that means believing that Jesus did not have the power through the Holy Spirit to save, therefore one has cut off their access to salvation. I think. I am not all that sure about that, I might be wrong.