Hello,
Denial of Service (a.k.a. DoS) detection is a firewall feature of SEP Enterprise Edition (EE). If you have SEP Small Business Edition (SBE) you cannot see this feature and it should be disabled.
You can enable/disable the feature only in the EE version from the firewall policy:
1. Edit a firewall policy.
2. Go to the "Protection and Stealth" settings.
This issue has been fixed in Symantec Endpoint Protection 11(11.0.6100.645) Release Update 6 Maintenance Patch 1 (RU6 MP1) and above.
OR
Please upgrade to latest version which is SEP 12.1.
Check these Threads below:
https://www-secure.symantec.com/connect/forums/only-client-blocked
https://www-secure.symantec.com/connect/forums/help-denial-service
Hope that helps!!