When I see this, I usually remind the senior students that it is easy to resist when you know what technique is going to be applied by a junior student, and that he/she should try to provied a more realistic attack. I also will quote O'Sensei's rules on training. More to the point, that training is not competition nor for gratifying one's ego. If this still doesn't work...the brisk ukemi drill is sometimes nice too
I will tend to remind them of the philosophy underlying aikido...and that until they try harder to grasp it, advancing will be difficult. (A person like this often is very concerned about rank and advancement).
Just my humble opinion.