pfSense feature requests should be added in Redmine in general: https://redmine.pfsense.org/
However, no this is not directly possible. pf, the packet filter used by pfSense, is a layer 3 filter it cannot filter by MAC address. So to do this the firewall would have to set a static lease for the MAC to get a fixed IP to use automatically and that would present a number of challenges.
PM which one is your fixed one.. I will pm you the one that is on site I show it as bad on.. Which I is one of the dbs used?
Also I don't think its possible for us to remove something from the askimet block list.. I think when this came up before and I asked the power that be here, Im just a mod - I do a lot of clean up on reported spam, that sort of thing. But I have no access at all to the workings of the spam filter itself.