Your best bet is force diplomacy mod. It's a little complicated to get used to the first couple of times but afterwards it's as easy as activating the EB script.

I used it extensively in 0.8. The Aediu were sending full stacks of Lugoae and other crap through the Alps but i was expanding historically and got tired of spending 20-30 minutes fighting a full stack each turn, so i just forced them to ceasefire each turn. They broke it each turn of course but i made them accept again, obviously i couldn't conquer Gaul yet because it was only like 230BC or something. I did it with the Sweboz too, they were worse than the bloody Aediu... 2-3+ full stacks through the alps every single effing turn.

Before i discovered the force diplomacy mod i just used to build forts with 1 unit of cheap nasty lugoae in it and auto_win defender/attacker each time it was attacked and i sallied.

If you're re-writing history however, which is something i also do but only when i play as anybody other than Rome, then why not just conquer them and make their entire kingdom a client faction of type-4 governments?

Get to it! It would be most profitable, and many thousands of slaves too. Good for business slaves are.