Well, it depends on which kind of crasher they are. For example, if your wedding crasher is an old acquaintance who showed up with some mutual friends assuming they would be welcome, it might not be worth raising the issue. Would it really spoil your day to have them around? And would you like to be thought of as the person who kicked out a benign acquaintance? Save your karma and let it be. On the other hand, what if the the uninvitee is an old flame who knowingly showed up uninvited and unwelcome? What then? The key to dealing with guests who absolutely cannot be permitted to remain at your wedding (including strangers who you decidedly cannot afford to feed for free just because they turned up), is to act quickly and let someone you trust do the dirty work rather than do it yourself. This could be a friend, family member, wedding planner or venue manager; let them take care of it and remember to instruct them to be discreet, polite and of course, firm. Remember, you’re not trying to make a point or an example of this person, you just want them out of there, and there’s no need to draw attention.
Many couples, especially high-profile ones, have started dealing with the issue preemptively by having guests present their invitations at the door before entering the reception. This is fine and can definitely be considered an option if you’re really concerned that your wedding will attract guests you don’t want. Just remember to inform your guests well in advance to bring their invitations, and make sure that the person checking the invitations is a member of staff and not simply a trusted friend or family member. After all, that person may feel too awkward to turn people away or might resent missing out on part of the party to check tickets. At the end of the day, wedding crashers are more often than not harmless and barely even noticeable so try not to overreact to them, and even welcome those who came simply to be part of your special day because they care. Avoid conflict when it isn’t absolutely necessary and above all, remember that it’s your wedding day, your top priority is to enjoy yourself, irrespective of a couple of wedding crashers.