there is more than one way
1. go to "Game Menu" -then-> "Options" -then->"Chat Settings"-then->"Chat: [put it on off ]"
2. go to "Game Menu" -then-> "Options" -then->"Chat Settings"-then->"Scale: [100% make it to 0%]"
3.go to "Game Menu" -then-> "Options" -then->"Chat Settings"-then->"Opacity...