problem with male mollies

i have 6 balloon mollies - 2 males and 4 females. lately one of the males has been very aggressive towards the other, keeping him away from the group! a couple of minutes ago i saw him chasing the poor guy and, well, chewing his gonopodium!!!

is there anything i can do to minimize this problem?

thank you!!! that's exactly what i thought i should do: getting a couple more females!!!