If the grief system is clever enough, it should be able to limit the potential damage of dedicated F2P griefing accounts.
- the type of weapon used
- the distance between the griefer and griefee
- damaging the same player repeatedly
- damage to enemies during the period of grief
- damage sustained by enemy fire during the period
- how recently the account was created
- how often the account is played
- the overall non-greifing damage to griefing-damage accumulated during a play session
The list could go on forever, but the simple basic fact is, we won't know how much of a problem griefing will be until beta (i know i personally intend to test this... exhaustively, eheheh).
The devs should then be able to respond with appropriate tweaking and updating of the grief system.