Hi Daniele,
the Paramater you search is called "Locktime" ( in German "Sperrzeit" ). With it, you can set how long a visitor is not counted "as new".
For Joomla 1.5 there exists a component to empty or reset the counter. In later joomla versions you must do it manually in the database.
Login in in your database (mysql) and select the counters table ( mostly jos_vvisitcounter ).
To empty the table ( not reset the counters max value ) use following SQL Statement : TRUNCATE TABLE jos_vvisitcounter
For a completly reset ( empty and reset max ) use following SQL Statement : DELETE TABLE jos_vvisitcounter
If you not sure then simple rename the table with your database tool (e.g. phpmyadmin) to e.g. jos_vvisitcounter_Backup .
With a new client request the counter creates automaticly a new table ( check Parameter "Create Table" ) and your original data stay in backup table.
Greetings
Micha
Ps.: have you seen
this Docu ?
( not in english, but you can let google translate the page )