Would a Baltic bastion built by Poland and neighboring NATO countries be useful?
Short answerYes, but its central task is protecting NATO's own shipping and supply routes across the Baltic, not simply denying the sea to Russia.
NotesExcluding Russia from the Baltic and making the sea usable for NATO are different tasks. Mines, submarines, and aircraft can deny an opponent access. Merchant shipping still needs protected local sea control, air defence, and surface escorts to carry supplies to the Baltic states. Swedish and Polish frigate purchases therefore support sea lines of communication and regional logistics as well as fleet combat.