An easy way of preventing mobsync.exe from running on system startup.

1) Within your group policy, expand User Configuration
2) Expand Window Settings
3) Expand Security Settings
4) Expand Software Restrictions
5) Double click Additional Rules
6) Right click and click new path rule
7) Add %windir%\system32\mobsync.exe
8.) Follow step 6 again and add %windir%\system32\mobsync.exe

If you wish to disable mobsync.exe locally on a machine and not via group policy, click here