Oh, they do, but it takes some time and slows down the entire simulation until they're dead. Can I do something to stop this from happening or make them die as soon as it does?
Don't start from hand-authorred bots. Start from a population of zero bots or at a minumum, a hand-authorred bot that is already cancerous.
Cancerous bots (bots that attempt to reproduce every cycle) are the norm in all long running evo sims but you usually only notice them early in an evo sim that begins with an artifical starting point I.e. hand authorred bots instead of zerobots. Hand authorred bots with hand-authorred conditional reproduction genes represent a lot of artifically stored nrg (body actually, since that is the key resource necessary for reproduction) that can suddenly be released by a mutation like a damn bursting and trigger a flurry of explosive reproduction. Like a damn, this hand-authorred code is totally artificial and ineffecient from selection's perspective. Not reproducing when you could gets selected against eventually. Naturally evolved bots will not store nrg needlessly waiting to reproduce. They will reproduce whenever they can (whenever they aquire enough body that the simulator allows them to).
Given the right environmental conditions such as preditor - prey co-evolution, we might eventually see selection favor larger bodies at the expense of more frequent reproduction. One can imagine conditions where having larger offspring is critical to survival and thus the storing of nrg (as well as the conversion of nrg into body) before reproducing might be heavily selected for in such conditions but that hasn't been seen yet, at least not outside of internet mode. It is possible the competition between different species in IM demands larger bots with more nrg reserved from combat and thus serves to provide enough selection pressure to preserve conditional reproduction logic, but this is largly conjecture on my part at this point. To my knowledge, we have yet to see a bot evolved from a zerobot ancestor develop any sort of conditional reproduction logic. (Doing so would qualify for winning the Conditional Logic Prize.)
If you want to stick with a hand-authorred starting point, you can use autocosts to jack CostX up when population spikes occur to keep it under control. You might also try the latest drop 2.43.1f. It's a lot faster for larger, sparser sims, but the benifits will be limited when you get dense clusters of cancerous growth. It will help, but you will still see the slowdown when bots reproduce explosivly. Eventually your hand authorred bots will all devolve and 10 million cycles down the road, your sim will be full of tiny bots that try to reproduce every cycle no matter where you start from. They will all be cancerous, but with no artifically stored reserve to exploit.