Only way to kill off a Waagh when it gets going is to defeat them in battle which kills their morale and gets them to desert. As you mentioned the trick is keeping pressure on them to stop them going Waagh in the first place. You need to stop them getting easy victories and raiding so build up town walls so there are no easy raids.
Ultimately though once they are live you just need to fight them, just go for the steel line/crossbow tactic and just hold with veterans/Lomgbeards whilst you rain crossbow related death on them. Their morale is so **** concentrated fire breaks them so quickly and if you have a lord strong enough to counter theirs you should easily win.
I had 1k vs 4.2k (ish) once and won killing around 3.5k of them and losing only a few hundred through the above tactic. Iron breakers or miners with charges are devastating on Orcs as well.
Thanks, starting to dominate now, only 3 more Ork settlements left. However, Arachon has just appeared at turn 131 and I cannot end my turn at all. I'm unsure whether this is because I didn't meet some conditions, I've not met either the short or long victory. When I played as the Empire I hadn't met the conditions (I don't think) but was able to continue. Anyone know what's going on here?