Again, I know others will have much better advice than I have because they have more experience, but the best way to stop this is to get them both fixed. Inside kitty might already be fixed and the aggressive actions of outside kitty are upsetting him. In this case, getting the outside kitty fixed will be the best solution.
I once had a cat coming around that would aggravate my two enough that they would attack each other since they couldn't get to the outside kitty. And of course this all happened at 2 or 3 in the morning. I solved it by taping paper over the bottom half of the windows so they couldn't see each other. This may be a temporary solution until you can trap and neuter the outside kitty.
Good luck....any chance we'll get any pictures of all these kitties you're talking about?
Tubby
Spring 1986 - Dec. 11, 2004
RIP Big Boy
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Peanut
Fall 1988 - Jan. 24, 2007
RIP Snotty Girl
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Robin
Fall 1997 - Oct. 6, 2012
RIP Sweet Monkeyhead Girl
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