For AP-450,1000 and 1500 is possible to clear community table and login to empty root account.
Just connect some pc or terminal to serial/console port. Then reboot router (You don't remember password, so You must turn off/on power).
Few seconds after reset on you screen you should see something like "Press ^C^C to ...". Please, press twise CTRL-C. Router should stop booting and you see boot promopt.
After you type "?" or "help" router send short help about available commands. Find command that clear community table and then do this and reboot router again.
After router come back, login on root and empty password - it should work. Immediately restore all accounts that you need and set administration password.
Piotr