You have to occupy every red circle to trap a ship. I've done that over 30 times and everysingle time the ship is sunk.

i have a question. did anybody encounter the problem, where you get a senate mission to block a port, but when you arrive there you see its actually blockaded by another roman family. you can't block it anymore nor get a reward. is this a bug or was i simply not fast enough?
Yes can be annoying but usually your'e safe if you have 2 or more turns to complete the mission as the A.I. rarely blockades for more than 1 turn. The A.I. only seems to stay if there are no other fleets in sight.