How do i get rid of this cat?

i have a cat who lives completely indoor he has a heart condition and is vey nervous . Lately this black cat has been coming to the door sometimes it just sits there but now more often than not its hissing at my own cat, and stressing him out which is not good for his heart!!! Ive tried throwing water at it untill it leaves but it just comes back :O argh any suggestions to what i can do will be very helpfull, thankyou