You could create a user group promotion using one of these to put inactive users into a secondary user group with a banner saying "Inactive" or something similar. Set the Display Styling Priority for that group to higher than that of other groups so that if you aren't using banner stacking it will display the Inactive banner instead of any of the others.
Problem in my case is that I've two UG. One is registered and the other one is registered plus that has many more privileges so I don't want to change the UG of that users, just changing, for example the user title to let anyone know that they're no more active.
All my users are in REGISTERED UG
All my pay users are in REGISTERED (primary) and REG+ (secondary) UG
The REG+ has more privileges
Problem is that I should create a third level UG but how can I manage the privileges here? If the user become from REG has some privileges but if he comes from REG+ it has more privileges so where do I've to put it?
That's why I'm searching for someone more practical like that addon...
OK, now I understand your set-up and that follows the guide that Brogan has written (assuming that the permissions set in the REG+ group aren't duplicates of any in the Registered group - there's no need to duplicate, only to add the extra permissions to the Reg+ group)
With the "Inactive" usergroup, there is no need to set any permissions whatsover, so leave it with everything set to Not Set (No). All you would be using the group for is to display an "Inactive" banner for the inactive users (by setting a higher style priority if you're not banner stacking) and using the Waindigo add-on to do this automatically for you. Users would be added to this group in addition to the other groups that they are already in, they would not be removed from any existing groups. So you would end up with some inactive users in Reg & Inactive and some in Reg & Reg+ & Inactive depending on if they are pay users or not.
Trust me, it'll work and it won't affect any of the permissions currently set up for existing users.