@MoatazKhan12 My understanding has always been that users added through a SIS enrollment must be controlled by the SIS. Meaning, no, they can't remove themselves via Canvas, they need to unenroll through the SIS so that their name is no longer on the SIS import. Otherwise the way most institutions have their SIS sync setup, the student would just get added back as soon as the sync happens again.
For your second question, if the student never accepted the invitation, they can unenroll because they never actually fully enrolled in the course to begin with. Many times the invitation is still at the top of their dashboard when they log in to Canvas so if that is still the case, they could accept it, then go in and unenroll from the course.
Not sure either of these answers are what you are hoping for but hope it helps!
-Nick