go to Policy -> Log Handler -> Default -> Access.log.
Whenever a user accessed a website this rule set is executed. It should have a log rule at the moment. You can put a rule on top of the log rule which says something like "If Authentication.Username equals Andre Then Stop Ruleset". If the username is "Andre" then Stop Ruleset is called and the log rule will not be executed -> No entry in the log.
thanks for your quickly reply.