Honestly, it is a question of good taste and courtesy. 99% of the time people stay within acceptable bounds in their roleplaying. For the very small minority who do not, the best policy is generally to just ignore them. They already look extremely petty in most cases. No need to lower yourself to their level. In the event that the RP has some political effect, then you may need to counter with your side of the story, while respecting the rule that Tom just linked: Game mechanics always trump RP. If a lord is kicked out of his lordship via a peasant revolt resulting from game mechanics, then that is exactly what happened. You may choose to RP varying reasons as to why it happened, but you cannot RP that it did not happen.
Granted I would have ignore his RP. Because he sent to all the rulers and attempt to sway other realms(some political effect), I have to come out with something. Whether the rulers accept our version or their version is another story.
As Indirik states, I try to create an alternate version of the events. In fact, the lands Nivemus currently occupied are devastated by the earlier wars and still recovering. Most of the peasants have flee the lands or died due to the wars.
Now his latest letter say all the tribes relocated to Perdan, another realm. Wow. No problem, just roll with it. I will not do any counter RP anymore to him. It beginning to get clumsy
