I do that even with my young students. If you don't want to do the "boring" task, you don't get to do the fun ones. I tell them so at that time, but with older gymnasts I'm not sure if I would or not, simply let it be a lesson learned and "shock" them a bit. With my little guys though, I make it clear that if they don't want to task A they won't get to do fun game B or skill B because with really young kids you have to let them know the expectations up front. With anyone over 6-7yo, I would just let them make their choice and then be faced with the consequence, hoping they would listen and work harder next time. Depending on age and reaction, I might also tell the parent what I did and why.