how do i stop my pumpkins taking over the garden?

Don't think there's much you can do about it this year - you could cut them back but of course you'll be losing part of your crop. For next year, to save space, you could train your pumpkin up a wigwam, rather like running beans, tying them in to help them along. This only works with small to medium ones - if you're trying to grow giants then this wont work for obvious reasons!