I have received a couple messages like this, but they aren't promoting telegram, instead they were inquiring for no obvious reason. Or maybe its just a hunch, I talked to them with professional behavior as I normally would and then they stopped replying.
But your case is different, if someone talks anything suspicious or spam that you might think is again the Tos, immediately report that account and create a ticket, my experience with Fiverr Customer Support has been really great. Customer Support will take actions, but they won't tell you what actions they're going to take.
In conclusion, if you see anything suspicious block and report them ASAP instead of continuing the chat. I hope this helps