I need help with UCAS choices?

I have applied to five universities and got two unconditional offers and two unsuccessful offers. One of the universities I want to go to is the one which I got an unconditional offer in 4 months ago. But I can't reply because my last choice is taking forever to reply (been waiting for 4 months). Is there anyway I can cancel this last choice because I'm not going to this university since I already have an unconditional offer to the university I want to go so this last choice is useless. I can't register or apply for accommodation until I reply to the offers and I can't reply because the university I don't want to go to is taking long/ waisting my time.