I do not believe that there are any restrictions on sending friend requests, so that would theoretically possible, with multiple accounts, although it would be rather useless as players can use /friend deny * to instantly remove all pending requests. With just a single account, however, I do not believe that it is possible to spam friend requests. Are you able to double check that /friend deny <name> is not working? To my knowledge, you shouldn't have to accept a friend request in order to clear it. If I am not mistaken, I have used it before with no issues; however, it was not extremely recently, and there may now be some sort problem preventing players from doing so.