The Rules Wrote:3.1 An attack is any hostile action that drains shields to less than 50%. Saying "Engaging" is not sufficient and aggressors are not allowed to destroy a ship before allowing sufficient time to respond. If a player is attacked he has the right to defend himself regardless of who is attacking. Trading nanobots, shieldbatteries or other ammo and equipment during a fight is also considered taking an active role in the engagement.
Scan the guy's B/B count beforehand. If he's missing some and your opponent looks refilled then torch him. Assuming you're a lawful like police/navy, he's also ignoring a lawful order to get out, which usually falls within your engagement allowances on your ID.