May you suggest me how to sketch a simple python script that prints in the central section of the galaxy
window a message like "you are not authorized to execute this tool" ?

This isn't possible right now, hence my reference to the open Bitbucket issue regarding this limitation.

The best you can do right now is (a) restrict tool access to non-anonymous users or (b) cause the tool not to run by manipulating the command line in the template and, by printing to stderr, cause Galaxy to report the job failed.

Best,