[bsloan1960]

It is management's responsibility to recognize and correct any hindrances to a diner's total satisfaction. They might consider carefully worded sign.
Such as:

"During Busy Times We Ask That You Make Your Table Available To Those Waiting Within 15 Minutes Of Finishing Your Meal"

Perhaps have it printed on the bill as well.

Maybe not that wording- I'm sure someone clever can come up with less confrontational wording.