I have read that wasps will not reuse an old nest so it should not be a problem. How big is the nest? Is it one of those that hangs from the roof like a ball? When I look in my loft I often see small old nests, about as big as a golf ball. I think they must have been abandoned unsuccessful nests.
When we moved in here at first the strange old owners had done nothing about a very big nest. It was in between the slates and under boarding in our out building (out building make's me sound about 95
). I used B&Q wasp nest destroyer on it. It is a foam you spray near the nest entrance. The wasps carry the poison into the nest and it kills them. You can get it as a powder as well. This nest was at least 2 foot square and about 3 inches thick within the roof.