I would not let users uninstall the agent manually.
With EPO 5.3.1 you can disable the removal of the agent with the command:
I do not know if you create a policy or a task for this problem.
Do you want to stop users from uninstalling the agent or do you want to remove the agent remotely ? Your question is not super clear.
When you use "FrmInst.exe /forceuninstall". it will remove the agent, even if there are managed products. If your users are local administrators on their box, there is always a way around.