I personally have never ran into this issue but I suppose if I had it I would just worry about the calcium and alkalinity and not do a water change until my magnesium level dropped enough to warrant a water change.
The only time I've ever had a true magnesium problem was self induced. I dosed magnesium itself by hand and over shot the amount that I put in due to my own boneheaded mistake by not doing the test right.
On the Red Sea kit when it tells you to add the 5 drops and mix for 15 seconds I did just that.
Your supposed to add 1 drop and mix for 15 seconds, then another drop 15 seconds.... etc.... etc...
That's what I did wrong.