Script to automatically clear User Manager log and session :

1. Clearlog:

:foreach i in=[/tool user-manager log find] do { /tool user-manager log remove $i; }
or you can make a new schedule instead of making new script and copy-paste these commands into On Event box:

/tool user-manager database clear-log
/tool user-manager database rebuild

2. ClearSession:

:foreach i in=[/tool user-manager session find] do { /tool user-manager session remove $i; } 

Set schedule for both scripts to run at designated time with interval 1 day or 2 days.