Thanks for your answer! I need something like that, so when I was searching on the web for this solution, Ive found the following page:

You can find the following excerpt, in "Sample file CleanupUsers.xml":

"...The result file also lists all users who have been muted, for example after too many wrong password attempts..."

I think this excerpt suggests that there is already such a thing in Websphere Portal. But I tried to find more informations, googling, and nothing! I think I will run that CleanupUsers.xml just to see what happens.