This is not possible in multiplayer (as I have tried it and failed) unless you want to piss off your mates by pausing, but
When fighting in a coalition war, be sure to peace out toward the end of the month, 20th or later.
After defeating the coalition, (they drop out) wait until a few days before the truce timer expires and they inevitably rejoin.
Single out a member you want to attack that you know would otherwise join the coalition as the truce timer expires.
Pause the day before the truce timer expires, unpause, (sometimes the -5 stab thing from trucebreaking still appears even if the truce is over)
Wait untill you no longer see that -5 stab on the declare war screen, and bring the hammer down. Usually you can beat the AI before they rejoin the coalition. I have done it numerous times in my Orleans game.
Alternatively you can just wait until the day ater the truce has expired, save the game and reload. The truce will now have cleared properly but the AI won't have rejoined the coalition, allowing you to pick whatever target you fancy. Obviously that won't work in MP, but coaltions are generally not the problem you'll face there.