This may work, or else it will make everything worse.Grief their geles with early bashes until they agree to make a deal to stop attacking with under x amount of toons. Only advice I can give you. We did this on Gwydion with a degree of success.
Stooping down to the lower level is a risky move - sometimes it causes the initial clan to back off, other times it just encourages more griefing and insults, which overall leads to a worse server environment for everyone.
A lot of it depends on why the one clan is griefing the other. If it's personal, griefing them back will usually only encourage trouble. If the one clan is competing fairly (in their eyes) in order to get drops, then they may be open to a deal if they are faced with more equal competition.