I'm not sure what fish that is, but I know when I had a severum with my Oscar they were fighting and wouldn't stop. I eventually had to get rid of the sev and it is a 140g tank, so with a 70g you may well have the same problem.
Have you tried completely rearranging the tank? That may confuse the fish into thinking they are somwhere new and having to re establish territory. Not promising anything, but it may help!!