Weddings are crazy and wonderful but stressful times as well. Sometimes someone ends up on your guest list that you wish wasn't invited, someone else invited, or invited themselves. Perhaps something happened between the time the invitations were mailed out and the actual wedding date that made you suddenly change your mind because you're afraid they'll cause a scene or complicate things. At times like these, it's better to just unvite them.
Talk to your fiance about the problem. Discuss the possible options and how uninviting the individual will affect other people. Will someone else cause a scene or try to ruin your day? If so, explore other options to resolve the problem.
Decide who is the best person to talk to the individual. If it's a family member the person who is related to them needs to do the talking. Don't make your future spouse tell your aunt she can no longer come to your wedding. Perhaps your parents or in-laws would be in a better position to do the dirty work. This is critical, so think hard.
Meet in a public place if the person lives nearby. If they are distant just call them on the phone. Start with typical greetings and exchanges but try to get to the point as quickly as possible so as not to lead the person on.
Explain the situation; the assumed invitation, the recent conflict, the potential for problems with others or whatever it may be. Be as diplomatic as possible. If they don't understand or are offended, tell them that you just think it would be best for the family as a whole or future friendship because any mishaps or tension during the wedding will just upset you.
Lie if all else fails. Lie, lie, lie. It's your day and this person might ruin it. If you don't want them there you need to make sure they don't show up. Lie however necessary. Then tell your other half about the lies so you can keep your story straight.