If you can get access to it, the foam spray works well - although it won't kill them all.
Wait till dark as they don't fly then.
I treated 2 nests with the foam - one was easy to reach, the second less so. The easy to reach nest - more or less got all of them, so was able to re-treat it and knock it down. 2nd was burried in ivy - so that took an attack with my jet wash. That said, some 'honeycombs' with the pupa remained, and I got stung removing the ivy as one or two buggers were still about.
If you can only see the entrance - i.e. flying in a brick gap, spray entrance with foam (again late on as there will be buggers queuing to land). Then sprinkle/squirt ant powder into the entrance, then the buggers will carry that in. That's how I got rid of one last year.
We've currently got one small nest in the eves of the house but I can't find it. I'm currently treating the entrance (when I remember) so that will get rid of them eventually.