It depends how those programs have added their context menu items.
I know that Tortoise SVN uses a shell extension so if you want to change it you'll have to use this method to hide/move/replace its menus, which is a bit of a pain.
UltraEdit might create a filetype context menu item, in which case you should be able to edit the name (and thus which letter is underlined and acts as a hotkey) via Settings -> FileTypes in Opus. (If not, you could hide its entry using the method mentioned above, then create a similar filetype context menu item which does the same thing and has a different name.)