Money sinks are the obvious answer, as you say. You can do this through incentives ("pay 1M gold for this cool new sword") or through regular losses ("your weapons and armor degrade over time, pay gold at the blacksmith to restore them").

You can also try to find core mechanics that don't involve infinite-spawn -> farm -> net increase in global wealth. Maybe the monsters are limited and can be over-farmed, for example, although of course that creates its own problems.

You can accept inflation as part of the game, understand the inflation curve, and modify your new-player experience accordingly (e.g. maybe you give new players a gift of Gold, untradeable of course, that is a percent of the total global wealth... so the more you have inflation, the more new players get to start off).

But mostly, this is an unsolved problem (in MMOs and in the real world).