Sales rep assignments and vacation/out-of-office

We need to figure out how to make round robin lead assignment not assign to reps that are on vacation or out of the office.  If a sales rep is on a 1 week vacation or even sick for one day, we don't want a lead to be assigned to that rep.  I've checked the help, explored calendar, looked at territories, etc and don't see anything in place to handle this situation.  I haven't checked if an inactivated employee will receive leads, but that would be a manual process that I would like to avoid.

Anyone have any ideas?

-Steve