I must be crazy to answer this at all, even with a negative response.
I see no foolproof way to do this. By setting your standard attack ships to only damage the planet 20%, they will do just that, then look for other targets. Finding none, they will then finish the job on the planet. Sending in plague-only ships will work but the losses will be horrendous and you will have to replenish the fleet regularly.
But face it, I won't give your buddies a chance to capture these planets anyway. Simpler to just surrender now.
Of course you could be talking about another game...
