Make Going Around You a Downgrade.

You can't chain a customer to you, and trying to breeds resentment. The real grip is different: you make the experience of working through you so much better than going direct that leaving would obviously make their life worse. They could cut you out - but why would they? Everything is easier with you in the middle.

This is the honest version of the moat. You don't block the exit; you make the exit pointless. The buyer who goes direct loses your speed, your guarantee, your single point of contact, your whole network of backups. Once going around you is a downgrade, the door stays shut on its own, and you never have to hold it.