the only thing I can tell you is that at some point everybody goes through this.
competitors are a$*holes... they are not going back down... just because they are doing something, they think they have some kind of patent on it and it only belongs to them and they are the only ones making a living, and if anybody else is doing the same they are stealing their profit.
it happened to me 3 times during last year, the only way I got them to back down was to do the same to them.
google can easily predict fraud clicks but they don't care at all, you either have to quit or fight back with the same weapons.
if they are making it hard for you to make money, make it hard for them as well. fight them back and after a few days send an anonymous email from a friendly competitor and let them know you are willing to stop if they are willing to stop too. otherwise, keep going. give them a taste of their own medicine.
I would love to hear a better solution, I can use it too. but I couldn't find any way to stop them.
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one easy solution, to make some profit while you are fighting this is that you use 80/20 rule.
since you mentioned you already ran this campaign so you have some data that shows you who is your most profitable customers (based on country or city). you can target only those cities and countries.
the chance of their bot having many proxies on the same city or country is very low. and even if they have it, you can block them which shouldn't be many. (most likely they are using global proxies)