Stealing Souls, Satan can do it..can we? Options

I am curious, not because I am a satanist but because I really would want to do this. I always here of people (usually satanists) selling their souls or making a pact with a demon or something but is it possible for another human to infact give away their soul to another? My reasons for wanting to know your opinion are quite private but I just am wondering what you think.

My definition of a soul is the entity in side us that separates us from lesser beings, it is not only are purpose but it is what enables us to CHOOSE what are purpose is. An animal has a certain role it is expected to play in this reality and it is extremely rare that it fails to do so, and if it does it is usually because of human intervention or circumstances that effect its environment.

Say a satanist gives there soul away, well in my eyes that person no longer has a say in what there purpose is in life. Satan from that moment on uses them and their actions to further his cause wether it be for their good or not. Same thing with christianity, even though it isnt as blunt christians in fact give there soul away to god...I mean have you listened to their songs? They practicly are begging to be his puppet....

Now in my opinion I do not believe its permanent, I believe that if said person has enough will power they can infact regain control of their lives.. but this feat would be extremely difficult to acomplish.