Not really. If you want it to have teeth, then it should come under a lawyer's letterhead, and that usually costs something (probably not much, for one letter).
It costs your reputation as a vendor which is permanent.
You don't threaten legal action against companies before calmly advising them of the situation.
You say that as if that is some bad thing. As a vendor you want to have a reputation for asking what you are fairly owed. The other option is to have a reputation for being a wet tissue anyone can walk through.
> You don't threaten legal action against companies before calmly advising them of the situation.
These are not incompatible with each other. Of course you calmly advise the company of the situation. 100%. You tell them that their 15 day trial period lapsed at <date> and that they continue using the <product> without proper license in place. You tell them where they can reach out to find the right licence for their needs. And you tell them that you intend to pursue them for damages if they remain out of compliance. All very calmly and professionally. Nobody is angry with anyone here. There is no bad blood. It is just a contracting oopsie!
When I was a teenager I would do super cut-rate work on computers for people, and my father did helpfully point out that undercharging for valuable work just makes it harder for people whose day job is to do the same work, because then they have to compete with a naive teenager. You're the kind hearted OSS / freemium vendor in this case. Threatening legal action costs nothing. Punishment is meant as a deterrent for antisocial behavior. Failing to even threaten them will result in less money going to people who deliver a public good.