The logic is time based, and is determined by the settings in the dashboard. The default is that if they make 3 failed attempts in 5 minutes, they get locked out for 1 hour. Therefore, deleting ip addresses from the attempt table that are more than 5 minutes old, or deleting them from the lockdown table that are more than an hour old, really will have no effect. Of course, in the future versions that logic will change somewhat, but I also plan to build in cleanup routines so you won't have to worry about doing it manually.
-Michael