What would you do if you had a disagreement with someone, to the point that you aren't on speaking terms and there is no chance of reconciliation?
Now this is actually an interesting one because it opens up all sorts of possibilities. Obviously, if people aren't speaking to you it usually can't get much worse (NOTE: does not apply to people who have access to your property, your parents, your employers, or sensitive information about you), and so technically you could do quite a lot to them. If they retaliate, they break the cold war and the battle moves into Phase II, and if they don't, then you could push it even further. Also a plus is when you share a lot of friends with this person, so that you frequently appear at the same events.
Here are a few suggestions of what you could do to turn the tables a little bit:
When you are in the same room, look intently at their face as if you were trying to work out a very complex puzzle. When they notice you, look even more intently at them. Follow up by shouting "Eureka!" excitedly, then return to normal as if nothing had happened.
Smile widely at everyone in the room including your opponent. If they respond, immediately frown and look away.
When alone in a room with your opponent, sniff loudly as if a strange odour is present, and look around the room trying to identify it.
Accidentally step on their toe and don't apologise.
Look intently at whatever they may be reading.
When you bump into each other, suddenly become very quiet, as if you were caught talking about them.
Text them at 4am as if you were drunk-texting your best friend and accidentally sent it to them.
Arrange a surprise birthday party with all your mutual friends and make sure that everything is terrible (venue, music, drinks, food). Then get all your mutual friends to say that they enjoyed it very much and make sure your opponent knows you arranged it.
At the same party, stand just behind your opponent throughout the party and pull funny faces when someone takes a photo. Inappropriate noises are a plus.
On a social, buy everyone a round of drinks including your opponent. See if they accept the drink.
Spill said drink on your opponent, then carry on as if nothing happened.
Come in one day and speak to them happily as if nothing had happened. If they respond, say that you have temporary amnesia but that doctors expect you to recover your memory at any time.
Shout "ALL HAIL BEELZEBUB!" loudly everytime your opponent opens their mouth to say something.
When they finish a sentence, applaud loudly as if it were the most moving speech you heard. Follow up with an enthusiastic “you really fooled them this time Beelzebub!”.
Talk to your mutual friends about all the good times you once had with your now-opponent whilst completely ignoring them.
Approach your opponent menacingly while brandishing a sharp object. When you get too close for comfort, drop said object, look confused, and apologise profusely while explaining that you are on a new form of anti-schizophrenic medication and that you haven't gotten the dosage right.
Talk about TV shows or movies with characters who share your opponent's name. Bonus points if said character is a porn star.
Spam their account with random emails all starting with the words "YOUR MOMMA!".
Follow them around the room while blowing a trumpet and singing the national anthem of the former USSR.
Every time they stand up, do the MC Hammer dance, complete with lyrics and tune.
Loudly hum the Darth Vader theme each time they enter the room.
Each time they leave, hum the funereal dirge.
Get a pet and name it after your opponent, then show it off to all your mutual friends.
Custard pie them in the face.
On a more serious note, a friend of mine has recently been abandoned by her "friends" when she needed them most. This post is dedicated to those whom we would call weasels, if only the weasels didn't protest on the grounds that they didn't want to be associated with these people.