How did you originally install the mplayer channel? If it was by using a WAD, you need to put that WAD on your SD card again (in the folder "wad") then when you open WAD Manager and select to load from SD, it will appear there as a choice. Instead of hitting plus (+) on it, which is for installing, you should hit minus (-) to uninstall it. Then you can delete the WAD if you want.
You know...if you want to delete a channel...you could delete it just like any other Wii channel. =] I forgot about that. Go try that, go to your Wii Menu and delete the channel from there!
Yeah, but it seems that this would work for the person, I've never had a problem...only time it bugged me was when I tried to reinstall Paper Mario later but it still had the ticket so I removed it and reinstalled it and realized...it took my save file away too! O_O