You can access the settings by navigating to Web Filter > Policies > Blocked Search Keywords
The Blocked Search Keywords page is where you define words the Web Filter module should block when a user attempts to search for websites to visit. If a user’s search text matches an entry in a blocked keyword list, the search query is blocked and the user is presented with a blocked message page.
Use this page to add and update lists of inappropriate search keywords. This is particularly helpful when you have users searching for keywords that are not automatically blocked by the Web Filter. You can use your reports to identify which keywords users are searching for in order to block the inappropriate ones.