Context: the manager of a boxing gym is arguing with a strong but problematic boy (問題児, he is arrogant and often violently beats other trainees) named Kamishiro and his trainer-sister that train in the gym. It is not clear (at least to me) if he doesn't want them in his gym anymore, but he says that his gym won't support them in their next match:
In the end, the manager and other guys from the gym show up in the locker room the night of the match, but Kamishiro doesn't look happy to see them:
Manager: おちょくるのも大概にしろよ…… 神代オオ
Kamishiro: 有難メーワクな支援 拒否るのにどこがおちょくってるって？
I understand that the boy is rejecting their support, but what exactly 有難 means here? Why is it used without い or く ending? I think the missing ending should be く, so my translation attempt is:
I am thankfully rejecting your annoying support, in which way am I making fun of you?