You don't - you can avoid the full curse by destroying the force field properly and defeating Belphegor while the curse is still incomplete, but once you've got it there's no way you're getting rid of it in-game.
How did he destroy the force field correctly and how did he defeat Belphegor? Because I assume that since you say that the full curse can be avoided is why I will inevitably have it, and if you tell me that I have to defeat Belphegor when it is still incomplete is why in that period I can get rid of it, Please answer my questions If you do not mind .Sorry to reply after so many days but I'm not very active on this account. First of all, Thanks