Further to shamrock's and Sir Degrader's posts that recommend Dispel Magic and patching (consider
Baldurdash too), try Remove Curse too.
I'm assuming your mage isn't a Wild Mage (a class from the Throne of Bhaal expansion) or that this problem didn't occur in Watcher's Keep (another Throne of Bhaal thing). I'm also assuming that you've slept / rested for at least 8 hours in order to try to overcome this issue.
Try (enabling cheats and then) Ctrl-R for an insta-heal.