Killing it totally isnt the best idea, as its often the only way small planets can inflict revenge on people who bash them.
Take, for example, a poor old zik who's FI and CO i killed because he was too stubborn to send them away - yes, stubborn not inactive
. All he is doing with his planet now is covert opping me, which i think is fair as its only a minor annoyance to me, whereas his killed fleet is significant to him.
As for 'many multis use cov ops to annoy top players', i dont know about that. If you suspect that they are a multi, report them. If they are a legitimate player, then still why do you care? covert ops doesnt inflict much damage to you at all. If you dont want to take the 'damage' from covert ops, build Security Centres to 16% and Eng priority to 2 and you have (virtually) 100% immunity.
most 'top planets' have full immunity anyway.