Sometimes you need to manually remove a custom field type that is corrupt, has been custom developed, or perhaps downloaded from CodePlex without any proper method of uninstalling. Hey, it’s open source, right? Can’t expect fully featured object removal.
After confirming the field type is not in use, you should be able to remove it from the xml in the hive (…\TEMPLATE\XML\fieldtypes_customtype.xml)
Go ahead and open the XML file before deleting it to confirm the contents are your custom field type in SharePoint and not something native to WSS or MOSS.
You will need to do an IISRESET after this file removal.
