This has happened to me a couple of times and I finally came across a method to resolve it.
When creating a new feature in Visual Studio I came across a problem where I changed the Feature ID of a Feature after it had already been deployed.
When I tried to redeploy, the installer process was not able to retract the old feature (as the id's had changed) and so carried on installing the new feature with the new ID.
Unfortunately since the old feature had not uninstalled properly, I ended up with all manner of problems and could not force a delete of the feature as the feature folder didn't exist any more, I didn't know the old Feature ID and there is no enumfeatures command in stsadm.
What you can do however, is look at the HTML source of the manage site features page (/_layouts/ManageFeatures.aspx?Scope=Site) and do a search on the friendly name of the feature. Just under the name you will see a div with the Feature ID and you can then use this in STSADM with the -force parameter to get rid of that little blighter once and for all.