Is it possible to disable a default keyboard shortcut for a special character, without the use of third-party software?
As a workaround, since I know you use FastScripts, here's a suggestion.

Undo the changes you made in System Preferences > Keyboard > Application Shortcuts > All Applications, by first changing ⌥M back to ⌥⇧⌘V, then delete the Paste and Match Style command you added. Note that you could probably just delete it without first reseting it to its default key sequence, however I do it this way to insure it gets reset properly.

With things back to normal, create an AppleScript .scpt using the code below, saving it as Paste and Match Style.scpt in the Users Script Folder (~/Library/Scripts).

tell application "System Events" to key code 9 using {option down, shift down, command down}


Note that you can also use keystroke "v" in place of key code 9, I just prefer to use key code over keystroke most of the time.

In FastScripts > Preferences... > Script Shortcuts, assign ⌥M to: Paste and Match Style

Now you will not get the µ character when pressing ⌥M when FastScripts is running.