Chunking tires can be a big problem...
If you are using additive, don't use the tires more than 1 or 2 times per day because they will became to soft and will chunk easy.
Check the edges of the tires so they are glued to the rim before you run them. It's very common that the edges of the tyres come loose from the rim after a few runs. If they are loose, just use contact cement or perhaps CA glue
The third advice is perhaps more difficult... Drive more carefully - foam tires will not chunk if you don't do any mistakes...
But if you do chunk your foam tires, don't throw them away. You can glue the chunk back to the tire with contact cement. It will work just fine!