http://it.gamerz-bg.com/index.php/moving-or-deleting-the-sbs-monitoring-database-in-sbs-2008/

Stop any backup you may be running in SBS 2008

1. Go to Services ( Run -> Services.msc )
2. Once the Service window opens scroll down and stop SQL Server (SBSMONITORING) service
3. Scroll down and stop Windows SBS Manager service
4. Now navigate to C:\Program Files (x86)\Microsoft SQL Server\MSSQL.1\MSSQL\Data
5. Now we have a couple of options based on what we want to do
   1. If we want to flush the database and start with a clean one of a normal size you can delete the SBSMonitoring.mdf and SBSMonitoring_Log.Ldf files. After they are deleted we need to replace them with the same database files but files but emptied. You can download them from here. This is all you need to do and can move to step 6.