I understand what you are trying to do, but I don’t think delete cookies on this forum, using phpBB works as you think. I’m no expert on phpBB but I found this on a forum for developers
Its mostly a user convenience. If you delete all the cookies set by the board, the board will simply give them back next time you visit and/or login. The cookies just help the board determine who you are any subsequent visits you make the board. If you have persistent login enabled on your board, it will allow the user to visit your board and never have to relogin, assuming they are using the same computer.