Hummm, they are having a party and didn't invite you? You could always hire some gangbusters to crash the party - oh wait, you called the cops and it's their job.
I have a very low tolerance level for others invading my space and I would consider a loud party invading my space. I honestly don't know what I'd to, it's been so long since I had a neighbor close enough to disturb the peace and quiet of country living. The worst I have to contend with is the neighbor's lights at their gated entrance.
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I'm sure that I'd complain by midnight on a weeknight and by 1:00 am on a weekend. By that time the party should be indoors and the music toned down. The best way to stop the problem is to call the police, file complaints, etc. EVERY SINGLE TIME. Eventually the message will get across that you are serious about the noise level and perhaps the police will take action.