Microsoft provides a tool from the Office family which is called MSI Cleanup Wizard that is often very helpful in removing registry entries for an application which became corrupt and cannot be un-installed via Add-Remove Programas | Software.
However, as the FSA Agent also goes through the routine that installs the minifilter drivers I would advise you to try and re-install the FSA Agent via the MSI. By re-installing you should get the Agent in a normal state that can be un-installed.
If you have upgraded EV to 8.0.4 you should try to re-install the Agent either via the VAC (push-install) or by copying the new MSI binaries to the file servers and deploying locally. This should then allow you to un-install if that's the intention.
Let us know how you get on