That is exactly what should be used as punishment.If I don't sign him up, my ex will and has. He tells our son and me, that taking him out of a sport should not be used as a punishment or alternative just because his grades aren't up. He tells me the child is "just a kid, let him be a kid" or that I should "stop trying to run everything, he'll be fine when he gets older".
Look bottom line is if dad signs him up for sports on YOUR time you are not obligated to allow the child to go.
My problem is that when he "gets older" I don't want to have a son who can't get into college because he can't spell or do math! I think my only recourse is to take him back to court and let a Judge have a chat with him.
Then put your foot down and stop with all the extra corricular activities.