OpenAI might in fact be a good target for stuff like this at the moment. Even if your argument is weak, they may be eager to settle generously if your suit threatens the speediness of their IPO in some way. But I happen to think this is in fact a reasonable argument: I put up a sign that says not to do something with my property, and you went ahead and did it anyway, costing me money. IANAL but seems like a straightforward tort, no?