Well I guess that in this situation (playing as Aedui and blockading northward expansion for Rome, one of the richest, strongest and most aggressive factions in the game) Force Diplomacy will be pretty useless.
All you will get is a War - Force Peace - War etc. thing every turn. Force Diplomacy is only effective if you have no or just a small common border or if you play as Rome and force peace upon barbarians who will be much weaker than you anyway.