Pyro / Poisonman / Swapper, Pyro kit (if they have fire resistance, it's useless), fire allows them to run slowly cuz of the damage taken every tick, and you can hit them many times more than normal, same for Poisonman, I think PMAN deals true damage which makes it more useful than pyro, otherwise if they have instant heal/regen pots then I recommend just make swapper holes before they rush your nexus and void swap them, or trap them on wool (if you're confident they won't have shears), then you can take their gear without them falling to the void.
If you don't have any of those kits, if they are going underground (like an underbridge) or the normal bridge, try to knock them off with like Sprinter kit knockback stick, Hero/Newb kit with their knockback stone sword, or just combo them with any kit and go demon mode and hopefully they wont lagback. You can also use Stopper, confuse/blind/slow them then let your teammates combo him until he's dead or voided.
Other than that, just try to make his health as low as possible until he retreats and get him off your nexus asap and let your teammates try to take out their nexus, but if he's just killfarming, there's really nothing you can do about it because they can retreat whenever they want to.
(P.S. : you can use Spy kit and also knock them off the void without them noticing you)
Hope this helps and have a nice day.