Hi,
This article can help you but issue may reoccurred.
Symantec Endpoint Protection Manager (SEPM) Database Transaction Log (sem5.log) Consumes Hard Drive Space in SEP SBE 12.1 RU1
http://www.symantec.com/docs/TECH178718
First you should upgrade to the latest version i.e. SEP 12.1 RU1 MP1
Upgrade path is SEP 11.x / 12.1 RTM --> SEP 12.1 RU1 --> SEP 12.1 RU1 MP1
No need to reinstall SEPM in your case.
We have a tool to shrink the database but I would suggest follow the above recommendations,
Shrink the Symantec Endpoint Protection Manager 12.1 embedded database
https://www-secure.symantec.com/connect/downloads/shrink-symantec-endpoint-protection-manager-121-embedded-database