Ask HN: Someone is using our trademark to market our competitors site

A friend of mine and I built a product called teamloop which is based around providing simple communication between coaches and parents/players. I currently have a trademark for teamloop in the U.S. as it relates to online sports communities. When we initially launched, the teamloop domain was already owned by someone else (but was not in use) so we launched at teamloopapp.com. I offered to buy the domain a couple of times but was turned down.

Now the teamloop.com domain is currently being used as an online community about sports and not only that they are actively using the "teamloop software" name to link to a competitor called TeamSnap.

This project has always been a side project for us as we both work full time but my question is what should we do? Should we send a friendly email to them asking if they would remove the links to our competitors site or should we ask them to remove the content entirely since it all seems to be infringing on our trademark? Should I just contact lawyer right away, or do we even have anything to stand on if I did contact a lawyer?

Thanks!