We don’t automatically remove computers by default but we do provide a tool that you can run which does this operation for any time interval you specify.
The easiest way I can think of to take a list of computers and then delete them would be to create an custom action and input the list using the third radio button for targeting (input a list of computers). Once the action is created, it will give you a list of the computers that existed in Bigfix and were also in your list on the ‘Reported Computers’ tab of the action. Now you can do a select all, right-click, delete to remove them. If any of the computers actually exist still they’ll report for the action.