The correct way to handle abusive players is to report them to the moderators and to block them. Blocking an user stops them from contacting you, and you won't be paired against them using the matchmaking system (you can still be paired against them in a tournament).
You can report them either by going to
en.lichess.org/report, or by pressing the triangle in the top left corner of their user profile.
You can block an user by pressing the stop sign (maybe its more similar to the no entrance traffic sign) on their user profile.