Radar wouldn't work. Most likely they approach the ships in Zodiacs, or other similar boats, which are far too easily lost in the clutter of waves, assuming they generate any signature at all.
I would think that a convoy system would work well. Certainly it would cause all kinds of problems for the ports, but the cargo would get there safely. Even one warship would be capable of protecting a large number of commercial ships by using their helicopter.
Personally, I would like to see the US and British navies get more involved. Have the UN set up a zone where UN countries would have the power to search any and all vessels for weapons. For ships... reactivate some of the old ones the Navy has in mothballs at the old Philadelphia shipyards. Technology doesn't matter that much here... as long as there are sailors to man them. One 5" shell from a Korea-era destroyer would ruin anyone's day in a Zodiac.
The last I heard about the Ukrainian ship is that it was being watched by warships 24/7 who would fire if the pirates attempted to unload the tanks.