How about asking them to behave respectably. A maintainer takes up explicit and implicit responsibilities to their users. If they are unwilling or unable to discharge those responsibilities in a respectable (“professional”) manner they should let someone else take up that mantle who is willing and able. Money changing hands is not material to this question, as not all responsibilities need to involve the exchange of money.

Just like we expect our neighbors to be respectful to us even when we don’t pay them for the privilege, there’s nothing surprising that users ask a maintainer to be respectful of their time. Yes, the users can go somewhere else, just like how we can move if we have a bad neighbor, but that’s just not a good way of going about things.