I was actually in the situation of being asked to intervene in conflicts many times during my time in Illy. Most of the time I gave suggestions about how the player who was asking the question could handle the situation.
A sampling of my responses in approximate order of their frequency are:
1) Put a rune up! (probably used in 50% or more of cases -- I know, people get tired of me harping about runes)
2) And what did YOU do to contribute to this situation? Try not doing that anymore.
3) Just ignore him, he'll probably go away
4) Join an alliance -- perhaps a training alliance if involved person were a newb
5) Use the ScottFitz model. Build up your city and pwn his ass(ets). (In reference to SF's adventures as a newb in Illy, not to anything more current.)
6) Move somewhere else
7) That person has done this before and I don't like him anyway -- join my alliance so we can beat his ass(ets). (I don't recall any specific instance of making this recommendation but I know I was tempted to do it at times! In general I required people to resolve any outstanding conflicts before joining an alliance in which I was a leader, as those who joined from such situations can attest. Possibly having me contact another player on behalf of someone wishing to join had the effect of quelling disputes or making someone more likely to come to reasonable terms. However, this situation did not occur frequently.)
8) Smack him back, he'll probably go away.
9) Ask someone else, I can't deal with any more of these situations today!
9a) Go ask Harmless?