Enalais and Ionion have gotten the two most basic rules of PvP combat.

Expanding on that toward basic guidelines: it's always a good idea for the two or more roleplayers in combat to contact each other and make an outline of the battle beforehand. As this isn't a tabletop roleplay with dice rolls or some kind of Video Game with its own built-in combat system, some discussion is always highly recommended. This makes for smoother roleplay and allows for longer, multiple-action posts.

It is slightly different in relation to GM controlled "NPC's" and Player Characters, as an NPC or set of NPCs are technically a "public use" set of characters. However, the owner/creator of the NPC always has precedent over their property.

Ultimately, Freeform Play-by-Post RP functions based on an honor system. It's expected to attack within reason or capability, and it's expected to receive those attacks within reason or capability. In other words: stick to what your character(s) know and can see, have your charaters do what they're capable of.