yes, I'd say they are called 'sempai', even if bully or other terms might also be appropriate---and I'm sure some other terms are in this case. I've got a few like that in my new dojo, but if the definition of 'sempai' is a senior student to whom we owe respect for their experience, then sempai it is. Besides, I learned a lot from my reaction to people like that: why get upset (and i did, too) when they purposely block your technique, often through an insincere attack? I smile, thank them, and get set in hanmi to try again before they can launch a 'lesson'. I may not get much chance to practice, but no tempers flare on either side, and self control is my main goal, anyway. and without someone to bait, bullies usually start to behave themselves, or move on. If they insist on my doing something not shown by the instructor, I ask them to please demonstrate instead. By this point, the instructor usually comes over and lets them know if they are leading me astray (they usually don't miss much of what goes on on the mat). People with supersized egos are great for helping me practice the best technique of all---peaceful coexistance without surrender.