If you won’t guard your own hours, don’t be shocked when everybody else spends them.

Unclaimed time gets claimed. People are not being predatory when they fill your week; they are responding to what looks available, and an open calendar reads as an invitation. Nobody else is in any position to know what those hours were meant for.

Put your own commitments into the diary before other people’s arrive, with the same solidity as an appointment. Learn one short sentence for declining that carries no reason with it. The first few refusals are uncomfortable and every one after that is routine.