@ TinCow,

As for not Exterminating – that’s good, Econ21 has suggested that your only income from this foray into Gaul might be Slave Trading which is a bit harsh I feel.

I think, that you should be able to demolish certain buildings whilst keeping Gauls ability to train troops – just select non-troop related buildings.

Temples/Shrines (certainly good Role play value in this one – “kill the barbarian Gods!”)

Any Sanitation building?

Other Ancillery buildings?

I’m not familiar with what buildings the barbarians can build but there should be 2-3 which can be taken down in each settlement. Sure it may effect upgrades (Armouries for example) but won’t prevent troop production.

To be honest Enslavement will effect troop production anyway as you’ll be taking a good proportion of the population away anyway.