My suggestion would be to either A) ignore and see if they go away, and/or B) have a domain name lawyer send a reply showing you won't roll over. That's usually enough for parties like these to back down.
If you need a lawyer recommendation send me DM me on Twitter (@marckohlbrugge). More than happy to help.
FWIW, some people might mention a procedure called 'UDRP' which is typically used by TM owners to quickly and cheaply get a TM-infringing domain name. However, one of the requirements for a successful UDRP case is that the TM needs to have been issued BEFORE the domain name was registered. Which is not the case here. So a UDRP approach would fail for them.
The other way for them would indeed be to sue and try to claim TM infringement in court. I don't have much experience with TM law, so I'm not sure what your odds of winning are. But there might even be a way to get their TM invalidated.
TL;DR: Talk to a lawyer.
P.S. If they have money for lawyers, they might also have money to buy your domain name. I wouldn't propose a purchase until you've talked to a lawyer (as it could hurt your case), but it's something to consider for later on.