How do you handle unexpected guests who show up on the wedding day?
Assuming your surprise guest is truly unexpected (rather than unwanted), I would apply the "Golden Rule." Strive to greet this person as graciously as your expected guests. The most polite thing is to simply accommodate this guest kindly and without any dispute.
If the scenario is one in which the seating and meal are maxed out, suggest to the newcomer that they return after the meal to enjoy the dancing portion of the evening.
Now if the guest is truly unwanted (such as a bitter jealous ex-boyfriend or delusional third cousin once removed) then your best bet is to let someone else handle the situation directly. The bride and groom should choose either the planner or caterer (or even the best man) to handle the situation. In a worst-case scenario, the facility manager or security should handle the matter.
No matter what, an unexpected guest should not ruin your day.