Is this on the SEPM or SEP client?
Meaning you cannot uncheck the option or...?
If on SEPM, it was a bug in 12.1.5 so you need to upgrade to 12.1.6
Cannot uncheck FileCache option on SEPM
Fix ID: 3640759
Symptom: When you close the Virus and Spyware Protection policy dialog, a redundant instruction saves the default FileCache options. Therefore, the policy does not save FileCache configuration changes.
Solution: Corrected the FileCache dialog logic.
http://www.symantec.com/docs/TECH230558