The following is from the user's guide on the topic of using HTML items. I think it also applies to the WebTracker item type:

Optionally, you can have navigation buttons appear while HTML pages are displayed. You can display any or all of: back, forward, home (will return to the starting URL), refresh and search. To enable this option, simply create a text file in your experiment folder called "browser.txt" and copy the following into it, changing true to false for any buttons you do not want displayed.

back = true
forward = true
stop = true
home = true
refresh = true
search = true