Vassals giving military access

[bradles0]

I recently released Byzantium as a vassal to nom those tasty Balkan cores, but they're giving ottomans military access into the Balkans (via giving serbia access), which makes me sad cause i want Serbia to do their reconquest undisturbed, and i don't want ottoman troops in the Balkans to respond to the Byzantine separatists i'm about to spawn.

How do I tell them to not give countries military access?

 

[CplKatie]

I would have released a single province to Byzantium then waited the 10 years to vassallize them so that you could still hang onto the full state of Constantinople.