Subterfuge!
When typical tactics fail, it might be time to shift things into a different gear. In some scenarios, it's simply impossible to play out a game normally, and while these methods aren't exactly honorable, I'll discuss them here anyway and have you choose your own moral ground. After all who am I to judge? I'm just another guy on the net.
The Monopoly Card
This is a personal favourite of mine, and I'm pretty weary of others using it on me. When you have this card and are deciding whether or not to play it, one method often employed to test the waters would be to offer trades. Offer trades for different resources, even trades which you might not actually be able to carry out. The point of this is to get a feel for what resources are available. If there is an abundence of one resource and you have a port, by all means play it and trade to your heart's content.
A variation on this would be to trade off all of a limited resource (which you own) at favourable rates and then playing the monopoly card to gain everything back, a truely diabolical way to make fast enemies.
The Robber
Another strategy involves the rober, whie users would typically target you if you're in a good position. One way to discourage them, if you know that they need a resource you don't have on hand, would be to reveal your cards. This sometimes works and you might be able to get them to pick another player. However, this can sometimes backfire and another opponent might then target you for the very cards you revealed.