You drill it into their fat f***ing heads in spring training that its unacceptable to show up grossly over weight and make sure to work them over harder than the other guys with conditioning drills. Threaten them with less privileges when the team is on the road, like earlier curfew, no fancy suites at hotels, ride the back of the bus, access to lesser quality spreads, kangaroo court fines for everyday their over their weight limit.
Point being there are plenty of things that can be done besides playing time/optioning them to the minors.