Concrete.

Or cleanup daily.

Or buy higher quality seed which the birds will eat rather than scratch out of the feeder.

Or buy seed preferred by the bird species you have in your region.

There is no mulch that will keep any seed from sprouting. A concrete pad will make cleanup a bit easier, but even there, given the right conditions, the seeds can germinate.

I put a tray under my feeder. The seeds that fall down, either I can place back into the feeder or some of the birds will eat from there.

I wasn't aware that birds might throw away some seeds they don't want to eat. Having chickens and quail, I thought the wild birds were just as messy about eating. Peck in and lots of stuff just fall out of the trays, though it would be stuff they normally would eat.

You could try adding chicken wire or an open hardware screening over the seed area so that the sparrows can't throw 75% of the seed out looking for their perfect bite.

Or i have seen where people take the large squirrel baffles and turn them upside down under the feeder so that it catches and funnels all the thrown seed down down to a cup or the like below it.