I've studied a lot of negotiation theory, and this is phenomenon that can't be avoided. The more often you deal with someone, the more likely you are to develop a relationship with them, be it positive or negative. You also discover their style of playing the game, like you would if you were a group of 7 friends that play the game every month, it's hard to avoid.

I think it becomes a problem when you consistently join games with people you have played before and the same alliances or belligerent relationships keep on happening.

If you're worried about it, stab your new buddy, keeps things interesting :)

Using DoS attacks to prevent a user from entering their orders is considered bad form.

Phoning them up at night and telling them you're had an emergency with your internet and aren't able to change them back, so you know it will look like a stab but really you don't want to make those moves is acceptable.

I'd say orathaic got it right. Playing dirty is a part of Diplomacy; however, you should play as that countries leader - not necessarily as yourself, at least to a certain degree.

That said, there's nothing wrong with bringing previous knowledge of players, but it's really about intent. The players intent should be to play that game well and should really not include aspirations outside the immediate game.

I guess the only thing you can do to describe what's kosher and what's not is talk about general ideas and specific instances - I don't think it's possible to have an all-inclusive rule or set of rules that really covers everything, since the key is - to a great degree at least - intent.

You are definitely allowed to learn strategic lessons from one game and put them into practice in another. You are definitely not allowed to take someone's grandmother hostage and threaten to torture her unless they support you into Munich. Between these two lines it's all a grey area - acceptability is also affected by ingenuity:
cf http://xkcd.com/654/