Moved to ePO for faster support
there is a sub directory under Common Framework, x64 or x86, where the frminst.exe is located.
Also take a look at the maconfig.exe, which can be used to manage the agent.
Could it be that you also have to add the switch, /remove=agent ? That's from the 4.x agent and I haven't yet tried it with 5.x but that could be helpful as well.