Hola
Please see this KB artice: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1035241
Converter agent uses a bitmap driver to track the changes on the volumes. In certain cases removing it requires restart of the machine. It is possible that's the reason you can't uninstall the agent.
HTH
Plamen