Adefensive player shall not: a. use his hands to add momentum to the charge of a teammate who is on the line of scrimmage. b. use his hands or arms to hook, lock, clamp, grasp, encircle or hold in an effort to restrain an opponent other than the runner. c. contact an eligible receiver who is no longer a potential blocker.

I believe the answer is: c. contact an eligible receiver who is no longer a potential blocker.

If the receiver is not making the effort to block the defender or has somehow past the yard line that is defended by the defender,  or move their way to the further from the defender, Contacting the receiver would be considered as illegal violation.