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.