So then, can those clans be used or not?
I'm confused by what you're asking.
Can an inactive clan's tag be used?: Yes, That's the point.
Can an inactive clan get their tag back from a living clan?: No, Unless they request it from the team, or jitspoe, But I suspect it will be: if it's been taken by a living clan, then you're SOL. And of course, if the tag stealers' clan goes inactive, you could take it back.
People are more worried about their members lists getting messed. Their clan would still have it's own page so that they can remember their glory. Basically, their old team would be untainted; they just wouldn't be able to recreate the clan with that same tag.
Maybe their profile would be tagged as 'inactive'?
Maybe on their profile (if their tag is taken) there would be an option to chose a new tag?
I will also say again, I think that if we give the ultimate deciding power to Jitspoe, he won't like the fact that it's one more thing he has to look in to on top of other things associated with this game. I don't know if his forum minions would be able to make these decisions (I don't see why not) But this warrants that someone actually checks the forums for requests pretty often. Using a set time limit removes all of this.
Also, I think I might adopt omni's email certification idea for checking if a clan is inactive; However only if there are 2 or less people in the clan after the X amount of days.