An alternative is to use the general's unit as reserves for other depleted units. Eventually his unit will be down to one (himself) and you can just disband him.

Personally, after the first 30 years or so of a campaign, I always keep at least one army of odds-n-sods to use as a mobile reserve (and short term seige army). After a battle they get moved up to the front line province, fill in any gaps in the main army and now the main army is back up to full strength 100/60/40/20 man units.