If you go through the GUI and are unable to uninstall a windows cluster chances are you have components that have not cleared themselves correctly from the registry. In order to get the target server(s) back to a pre-clustered configuration, you can use the instructions in the following link:

http://support.microsoft.com/kb/282227

Do note that this will forcefully delete all configuration in the registry redering any and all software that was associated with the cluster unusable.  At that point you ahve to manually clean up those entries as well.  So, use this process with caution.