Two suggestions I've seen so far:
1) Create an army for him, but don't assign any military units to it.
This seems very practical, even if it wastes an army command. Sending him WoW (Waaaaay out West

2) Deliberatly putting him in harm's way, hoping that he gets killed or looses seniority points as a result.
Admittedly, Mac was useful to me for organizing the AotP. But I couldn't believe how worthless he made every commander underneath him. I was curious if anybody else found any practical use for him.
bsz