Well it depends on what your theme uses as it’s sidebar container.
.bbPress is a class which is applied to the body tag so you can tell if it is on a bbPress page.
.sidebar is a guess at what the sidebar container is on your theme.
stack them together and then use display:none to hide the sidebar on all bbPRess pages.
You can add this into your bbPress stylesheet or your wordpress stylesheet (it is usually nicer to add it into your bbPress stylesheet so that all bbPress styles are contained in one place).
Good luck!
