Pick one night a week that belongs to nobody but you.

Time that has not been claimed gets claimed by somebody else, usually with a reasonable request at short notice. Deciding in advance that one night is spoken for means you are no longer weighing each ask against an empty evening, because the evening is already taken. That makes declining much simpler. You need no better reason than it being yours.

Choose the night this week and treat it like any other commitment. When something comes up for it, say you already have plans, which is true.