Should guild get disbanded if the leader of it hasn't been on for a certain period of time so that you can re-use guild names?  

You can submit a ticket to http://support.toonslab.com/ and ask them about disbanding specific guilds.

Normally the leader has to be inactive for a long period of time and if the guild has over a certain amount of towering hours I don't think they will disband it, but always worth a shot!  

thanks!