block any URI with status 404 or 403

I don't know if this is possible because the F5 would need to know about the page not existing prior to sending the request to it and since the server would be the one responding with this message it would require the request be sent to the server in the first place. The only other thing I can think of is creating multiple data-groups that have a list of every sites files and then match against that but that really isn't feasible. Any particular reason you want to block 404 and 403 outright without sending it to the server?