Three assumptions: 1) Relativistic speeds are possible, but difficult to attain in terms of reaction mass 2) Ultimately, scientific advancement does not reveal FTL, but does not rule it out entirely. 3) All intelligent life is toxic to its own environment 4) The heat death of the universe is unavoidable.

These assumptions are consistent with our known understanding of the world.

If a matrioshka brain can be constructed, the brain must choose between making more computronium, and launching a relativistic mass at a target. Furthermore, a matrioshka brain is by definition, very resilient to objects flying towards it at relativistic speeds; a nicol-dyson laser array can direct and incoming object into a stable orbit, into the star, or failing that, the networked matrioshka brain loses very little when a relativistic-speed object crashes into a small proportion of the total number of nodes. Reaction mass required to move computronium is probably better spent on 'making more computronium' so a matrioshka brain is unlikely to attempt to move out of its' own solar system, if it even has the option to do so, furthermore, any 'seed' creates more alien life, which runs the risk of pissing off other silent aliens, or, on a long enough timescale, creating a hostile alien.

So the brain's rules would be simple: 1) keep measurable signals within the solar system of origin 2) Send no relativistic probes that could reveal position to a potential life form with superior technology 3) repurpose all usable mass in the solar system, including usable mass entering the solar system from without, into computronium indistinguishable from a dust cloud, ensuring that non-matrioshka brain life will not be able to infest the system.