Mark Maintainer maintains raid marks on players and prevents other addons from changing or removing them. You need to be raid leader or assistant in order for the addon to work in raid groups.
How to use:
Type /markm to bring up the configuration interface then type the character name in the box next to the mark you want that player to have. Alternatively you can mark the people you want marked and press "Autofill" and the boxes will fill accordingly. The Clear buttons clears all fields and all marks set by the addon, and the Pause/Start button pauses (and restarts) marking by the addon.
If multiple people run this (or similar addons) but with different settings that can result in the addons endlessly overwriting each other. This addon has a built in throttle system to prevent spamming marks in such a situation. Because of this there is a small delay (<2 sec) before the addon restores a mark if it was changed.