Just click on the Avoid tab first, then scan down the list of stuff, click "Employee number" then add his, you may want to put him in your defaults as well, just in case you forget to bid one month.
The downside is, if he gets a great trip you also wanted...you won't get it, the upside is, you'll fly "Tool Free" on a bad trip.
I've found a Tool can ruin the best of trips, and a good guy can make you have fun on any piece of crap trip.
As I've said countless times; It's an easy job, yet some guys work so hard at Pucking it up.
It's supposed to be fun, right?