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Be civil and *document*. If it does come to court, your kind requests (in writing) to desist, as well as your flexibility in helping them identify and correct their violation will help make the case that you have taken the needed steps to protect your property without being an asshole. Their response, or lack thereof, will then be a building block in your case should it come to that.

Remember, civil court is usually just a judge and that judge may react somewhat differently if it is clear you are a jerk. While it is true that they may have to rule in your favor even if you are a jerk, they are unlikely to make it easy on you. There are many things judges can to to move things along that are at their discretion.

Treat every communication for violation as it it was going to be entered into evidence, but remember, court costs money and (more importantly) time. You are much better off if you are getting violators to comply without taking it to that level.

I've been involved in an open source project (FreeBSD) for a long time. There have been a number of complaints about GPL violations in the past. These complaints are usually made in private. That helps a lot. Often times the compaints are wrong (The GPL code that was alleged to have been taken and improperly included in FreeBSD turned out to have been taken from BSD 4.4lite and incorporated into the GPL code was the worst example). There have also been cases where the same code appeared in drivers in multiple places. Again, that wasn't a GPL violation because both places took the code from a common data sheet. Sometimes supposed violations are cleaned up out of an abundance of caution: it isn't clear the code is improperly included, but the code in question is easy to rewrite and/or icky to start with.

There are also times where GPL code is improperly imported code from BSD as well. Even when these are found it isn't always worth it to complain. Sometimes the gain from complaining is so small that it is easier to just let the folks use the code and not worry too much about it. Sometimes having the code out there and improperly licensed is better than getting it removed from the code base.

In general, I've found that most people that aren't lawyers don't know the law or the provenance of the code very well. By complaining in private, you get a chance to learn a bit about both. You also give people a chance to make it right. With large open source projects, the chances for accidental mistakes are high. The projects are generally keen to avoid the mistakes in the first place, and even keener on making sure that they get ironed out after the facts. Turns out most companies have a similar view and will do the right thing when asked (but sometimes it takes a little time, which is OK: the GPL never said instantly on demand).

Of course, this begs the question about the validity of the License to use GPL software after a violation has occurred, the scope to which license is lost, how to get it back, etc. GPLv2 is silent on the issue, while GPLv3 gives you one shot to fix it (but that's likely insufficient for large companies that have multiple product lines done by disjoint sets of people all of whom aren't educated on the finer points of incorporating GPL software into their products).