I have a very good solution: a garbage bin. It sounds harsh but after the second round of this well meaning relatives will either bring better toys or will stop giving stuff, everybody wins. Taking 'wrong' toys to the goodwill just keeps them in play and will essentially make them somebody else's problem.

And good toys (which tend to live a lot longer than crap ones) too can be taken to the goodwill later on and then will make someone else happy.