Open Terminal, and type the following:
qlmanage -m plugins
This will give you a fairly long list of all your active QuickLook plugins:
The types listed there might look a little strange, like
com.apple.safari.bookmark. Those are Apple
Uniform Type Identifiers or short
UTType identifiers, and there is a list of the default ones available from Apple here. If you're looking for a specific file extension, it might be a good idea to open that link and search the page for it. A search for
txt reveals its UTType to be
public.plain-text, and this is the type you will find it under in the qlmanage output.
So if a particular type of file looks strange in QuickLook, just look up its path here, go there and disable it (for instance by renaming its file extension to something other than
qlgenerator), then come back to Terminal and force QuickLook to reload its list of plugins by typing
qlmanage -m plugins again to make sure the offending qlgenerator is not loaded anymore. Careful: QuickLook will even load them if they're in the trash, which is why we renamed the plugin above instead of trashing it.
Open a Finder window, find a file of the right type, and hit the space bar to double-check.