To tell which are the girls, which are the boys:
Juvenile convict cichlids are monomorphic until they reach sexual maturity. The male is mostly gray with light black stripes along the body. Males are larger than females, and they have more pointed ventral, dorsal and anal fins which often extend into filaments. In addition, older males frequently develop vestigial fatty lumps on their foreheads. Unusually for fish, the female is more highly coloured. She has more intense black bands across the body, and pink to orange colouration in the ventral region and on the dorsal fin.
And to prevent they having sex... well:
Sexually mature convicts form monogamous pairs and spawn in small caves or crevices. In the wild, the fish excavate caves by moving earth from underneath large stones. Females lay the eggs on the upper or side surfaces of the cave to which they adhere.
So, perhaps, the best way would be separating males and females as soon as possible, and removing places where they would reproduce.
But, of course, this can have undesired effects: when there are few opportunities to reproduce, they might get even more active.
And some other solutions:
give all them to someone else who wishes to have those species;
begin to sell those little kids, and get a new aquarium :-)