Take them to the European Court of Justice and seek fines. They have ways to sanctions member states and now it's an easy bet they'll development more in an attempt to force compliance, especially with immigration policies.
OK, who’s going to collect the EU fines & by what means of coercion?
Close all roads leading in & out of Poland? That takes cooperation by EU members who don’t care about lost business and could be regarded by the Poles as a casus belli.
Total trade embargo? Punitive tariffs? Brussels may be forgetting that EU members are still sovereign nations. It’s not the EU Reich just yet.
If all else fails, they may attack Warsaw with the mighty Belgian Luftwaffe.