I've found it:

I've modified it in the file universe_wgsi.ini and now I can delete user by the admin tool panel. 
The problem is: when I delete the account and purge it why it still remain in the list ? is it possible to delete it completely ??

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From: leonardsquall15@hotmail.com
To: galaxy-dev@lists.bx.psu.edu
Date: Mon, 7 Jul 2014 09:49:48 +0000
Subject: [galaxy-dev] Delete an account (Galaxy local instance)

Dear Galaxy developers.

Is it possible to delete an account (user not active anymore) on a Galaxy local instance as Admin ?

Regards,
Pat

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