Can in-game purchases be blocked?

Create a new account, give it family sharing, make sure the new account doesn't have payment methods attached.

I would assume that'd do it- though many online games link to the specific steam account, so you may lose in-game account sharing. :X

I think this depends on the game and or plattform
Some have a different login for purchases....or sometimes there is a workaround,like blocking ports or store-url for the game in your firewall.