If it's outside and accessible put some pariffin in a sqeegee bottle and squirt it into the nest late in the evening when they've gone to bed, whether or not you set light to it is up to you. If it's in a roof or likewise let the experts do it, it'll cost you but it's well worth it.
Aerosol wasp nest destroyer foam used from 3 metres away from nest early morning or late evening when the blighters are at home. Available supermarkets - mine from Aldi @ £1.99.
Tell the ones who are scared of them, eh!?
All depends on where exactly it is I suppose. Just cover up well if you're going to do it.
I have a hornet's nest to do in a roofspace. I am borrowing some beekeeping gear for that.