In another life I was an event photographer.
I only had to do this once, luckily.
Owning the original (digital) image and camera secures your claim to be the owner of the image. Metadata is imbedded in your image, shot number, camera serial number, owner name...
The easiest thing to do is to download a "Cease and Desist" template, fill it in, and send it to the person who is using your images.
Of course, it's a lot easier to do this if you have the person's email.
Sending the letter electronically, taking a screenshot of the image, and including dates in the letter are helpful if you ever want to escalate. People I've spoken to about this told me to be corteous and friendly when doing this, since most of the time people don't know they are infringing, and will happily take down the images.