You have verious options at your dispopsal:
1) turn off chat and/or throwables altogether
2) Block the specific user (block chat)
3) If the text in the chat is truly offensive or hate filled, you can report it to
poker stars. such behaviour can be reviewed and may result in a suspension of their account
4) sit and wait and throw stuff back
5) realise that this is a tilt tactic they are using...and it is working! Read up on Tilt Control and practise some of the exercises. Find your Zen and then take him out... I often do this by limping AA/KK and then jamming on the flop.
Good luck!