As most said in comment section, in peace time they cant close it, in war time would be super easy, as for example saab RBS15 Gungnir anti ship missiles has 300km range, and closest two areas of finland and estonia is only 40km ish apart, and widest part 120km near russian borders. These can be launched from air, ships and trucks. Put these systems on both coastal areas and russian ships not coming thru. Sweden by itself could close that straight with its submarines, which are the stealthiest in the world. The whole baltic sea is not that wide for todays tech. The whole baltic sea would be no go for russian fleet. They would be attacked from all coastal areas.