This one. That's what the filing fees are for a lawsuit like this. There's no rule saying you have to pay a lawyer to write a statement of claim.

Edit:

Courts deal with contract law disputes all the time. It's their bread and butter, everyday, nothing special stuff.

Edit2:

To you below, citation needed

Is that also what it costs when you lose and the court makes you pay their lawyer time?

Use the CCB then?

Edit: I'm somewhat mad that there's all these tools out there to solve the screeching about GPL violations and nobody seems to want to use them.

	▲	apublicfrog 3 hours ago \| parent [-]
		For reference for non Americans/non legal people: > The Copyright Claims Board (CCB) is available to resolve copyright disputes of a relatively low economic value and provides an efficient, less expensive alternative to federal court. https://ccb.gov/