Enslaving doesnt work too well if there is a lack of controllability (via threat of easy overpowering and punishment, restriction on movement/action, longterm mental adjustments that prevent consideration of contrary actions, etc...) Caveman days was a little to freeflowing for alot of those things.
Caveman X is enslaved by Caveman Y and at first opportunity brains caveman Y with a convenient rock and make his escape. Not too practical. Stealing women might be more common but thats more adoption than enslavement.
BTW if multiple humans existed in the same local there WERE social conventions (even animals do this) so you might have to build up proper repercussion for 'taking things too far' (and making it plain to the player ) That is actually were the complexity explodes in behavior mechanisms where you move up a magnitude. To be closer to realistic there has to be 'history' not just reactions of the moment. Things happen as a continuum where an action has results/effects that may last a long time - the overall situation has to be considered in motivations and reactions you can get.
Another avenue may be people who want to join you (strength in numbers and survival dependant on sharing resources). A typical 'capital' punishment' in many nomadic societies would be exile where the environment was often such that it basicly WAS a death penalty. It also might make you look more than twice at someone who comes along solo who cant provide an adaqute reason who MAY have been exiled and might not be the best candidate to join you ....
Another consideration is balance. You COULD enslave that person but do you need them will they supply something you need or already have too much of. One guy has 10 slaves (with little autonomy) and unless he is mean enough to cow them all sufficiently one might get the idea that with a few others they might easily overpower the master. Too many women for one Caveman and it throws off the 'hunter gatherer' balance (he has to/cant hunt too much to maintain the nutritional balance to be flexible in the seasonal variations)
Something I formulated years ago - all social interactions are part of a negotiated agreement of some kind. " I dont attack you and you dont attack me" is one of the simplest. Another is "I join you and do what you say and I make use (and add to ) your resources."
It all is based on getting something you want/need and giving/supplying something in return that isnt too much.