A bulletproof method would be to create a new BO called userrightlevel with two attributes; status (unlocked,locked) and value. make default value of status='unlocked'. Then create your six instances of this object (0,1,2,3,4,5).
Change your SystemUser.UserRights to use 'Dynamic Choices' based on userrightlevel and filter the values with a query WHERE userrightlevel.status='unlocked'. When 0 is chosen for the first time, have a rule that sets the status of 0 to 'locked' and it will not show up as an available value in the dropdown list anymore.
There are almost always a variety of ways to approach a problem in AwareIM, you will just need to decide which is the best.
Pete