In the late 23rd century certain groups of mankind stared down the barrel of what may be possible in the future: cross galactic relativistic kill missile storms, gamma-ray Dyson Beams, Kugelblitz black hole weaponry and potentially even exotic matter-based weapons messing with space and time itself. Some even proposed that these weapons are the answer to the Fermi Paradox, a late and great filter. The view that any group potentially willing to use these weapons must be dealt with at any cost and as soon as possible became quite popular.
After the victory over the old baseline nations in the First War Of Sol, the trans-humanist groups started the gardener initiative, a program for swift, systematic interstellar colonization. All colonists had to agree to mild brainwashing that would instill disgust towards these exotic weapons of mass destruction. This was meant to instill a negative bias against these kinds of weapons in all the future civilizations they would seed.
Additionally, everyone signed a treaty and vowed to make every daughter civilization sign the treaty as well. The treaty states that:
no signatory shall build or use exotic weapons, however owning blueprints is fine
if an exotic weapon is used in a system, it will have 25 years to defend the incident
every system within 25 light-years will broadcast 50 years after the incident if they believe the transgressing civilization has culturally shifted to an acceptance of exotic weapons or if it was an isolated incident
if less than two-thirds of the surrounding civilizations believe that the transgressors are not guilty, everyone must commit enough resources to the purge, so that the transgressor's energy resources are matched five times over (there are official formulas for this)
exotic weapons may be utilized in the purge, which happens 25 years after the broadcast vote
Since the political state of the galaxy is relevant to this, here is a brief rundown:
most civilizations are gardener descendant, thus signatories
gardener descendants are extremely diverse, however most are trans-humanist and share a common origin
the few pre-gardener interstellar colonies have been "pacified"
one later colonization wave was CORE, digital minds with an efficiency agenda, they ratified the treaty
baseline humanity was mostly wiped out during the great genocide/ pacification of last regressive strongholds/ Second War Of Sol, only the Luna Hegemony and the Callisto (later Jovian) Republic survived
the few baselines colonized very little and with very little organization and signed the treaty due to outside pressure
mankind has spread across more than 80 percent of the milky way by now
life seems to be common in the universe, but mankind seems to be alone thus far; even distant galaxies seem still uncolonized
no FTL travel or communication, the setting is pretty hard SciFi
most interstellar travel is done via boost beam at 0.7c, however more advanced drives do exist
no great interstellar empires; generally each system is its own or several polities; some macrostates exist, however, they are rarely bigger than a few systems and rather loosely organized
Will this treaty be effective at preventing mankind's galaxy from descending into a state of eternal high energy warfare until everyone is dead? If not, how else could such a situation be avoided? Isolated incidents and the bashing of bad apples are expected, but escalation should not happen.
I'm aware that this comes quite close to being opinion-based, but phrasing it otherwise; I'm asking how to keep weapons of mass destruction out of wars in an interstellar setting.