T40 is certainly not an early rush.. If you are not going domination then my advice is to 1) don't scout so you meet him as late as possible (by that time he may have already found a different victim) and 2) the turn you meet, send a delegation, a free luxury copy or 30 horses as a gift for free, and open borders. This should bring him up to friendly and you can declare friendship before his agenda comes into play. Good thing is that for a relatively cheap price you permanently rid yourself of this threat for the rest of the game. Worst cases are when they meet you early and you have nothing to give them.