I'm going to say no.
Repeatedly attacking the same player without reason, that's when it crosses into the territory of harassment.
Would very much depend on the conditions though.
Let's say you are the attacker, and you see someone in the system, you say "This system is in lockdown, leave or be attacked". Then attack, that is not harassment. It might be a weaksauce roleplay excuse for PvP, but hey, that's not the question. If they keep coming back, then i still don't see it as harassment.
However, let's say there are other people flying in the system. If you are attacking them as well, that's fine. If you are not, and only attacking that one player, then its definitely in the realm of targetted harassment. (again, there might be exclusions, for example, powerplay alliegance).