Play the game.
Provided this is not a Tier-Dropping scenario, there are no rules about how many times you can hit someone.
Understand that this is a Player vs Player [PvP] Vampire game. We encourage people to have fun and embrace ingame deaths.
As this is the case, you can of course retalliate in kind. If you are unable to achieve a win because they are too strong and it's bothering you, try asking if they will join your Clan rather than retaliating. If that doesn't work, we suggest utilizing the Elders in your clan to mediate if necessary. Otherwise, there is the hitlist!
Keep in mind, the developers have designed the game so that you cannot lose more than 55 xp from a single player in a 24 hour period. Each time you are killed, you lose 1 less xp than the time before. 55 xp is very easy to make up, so while you might not like the fact that someone is attacking you more than you think they should, in the end, your loss of xp is protected.
However, if the same player is consistantly attacking from a Tier higher than yours, please visit the Community Support Desk (CSD) where you can report the incident by submitting a Ticket Request.