Manually deleting the program and VST plug-in configuration files will do the trick. You just need to find "MusicMaker.ini" and "VstPlugins.ini" and delete them while Music Maker is not running. Just deleting the VST config file alone didn't work for me in this version of MM, but removing both files helped. Run a Windows search for the files to find them. They're in the same folder on your hard drive. Here on my XP system and with default installation parameters, this is the right folder:
C:\Documents and Settings\UserName\Application Data\MAGIX\MusicMaker15\
On a Vista system it should look something like this:
C:\ProgramData\MAGIX\MusicMaker15\UserData\
You might have to reconfigure your Windows Explorer so it will show hidden folders. Then just delete the two *.INI files and resart Music Maker. The links for the undesired plug-ins will be gone. You'll just have to re-import the VST plug-in folder that you actually wanted to use with the program, since it would have been reset by this procedure as well (plus a few other basic settings in the program).