Your tenant and site item will have a field called
Role domain (in the
Security section). If you clear that field, the script will be available again on that item.
Be aware though that clearing that field will not remove any security (as far as I know). So the security settings created by the first run will remain and you might get some strange effects if you run it again. In order to remove everything you would need to remove all the created roles as well.
I would advice you to think about what you are trying to achieve and see if you really need to start over or just change permissions on your roles (which you can do without re-running the script). It might be a good idea anyway to create roles on top of the ones that SXA created (that's what I do) and use those for extra security settings.