If you are to try a blockade, I'd have to assume you'll bring enough matte with you to start dropping relatively heavy chunks of matter onto anything you see missiles being launched from.
Send a squadron of cheap robot ships that are painted in EM-absorbing materials, operate at cryogenic temperatures, communicate only on tight-beam lasers, and beam all of their minuscule waste heat in one direction. These ships would make a final course correction by ejecting cold mass, and take up station near the resource. When an enemy transport comes nearby, I'd send a command to activate one of my robot ships and have it launch a missile. Heck, since at that point it would be disposable, I'd have it spin-up to high-temp torchship mode and have it be the missile.
The enemy won't know how many of these things I have. However, the real purpose would be to get them to react and maneuver resources in place to detect my hidden ships, so that I could detect the energies of those maneuvers to set up the real attack.
