It isn't proven if space time excludes fractional charges. There might be truth in it. A great many particles are made of quarks and gluons, suggesting that if the particles were responsible for the exclusion then there might be exceptional particles that are not able to perform the exclusion process. Such particles are not observed.
One possible explanation might be that space time performs the exclusion, removing the problem of many particles all conforming individually.
CERN has reported making a plasma of quarks and gluons for a brief moment after collision of heavy atoms at immense energy, something like a big bang. The plasma quickly converts into a mix of ordinary matter flying in all directions.
Results are suggesting that quarks and gluons can escape from confinement briefly under the most extreme conditions during which they are not dominating each other. They can not survive separately in the free state.
Either the sub particles bring an end to the freedom, or the local space regains control and rejects the fractional charges. A sizeable number of sub particles are found, 6 quarks and 6 antiquarks, with three possible different quantum color charges. It seems like enough individuals to expect an exceptional case. None are found.
Maybe the best clue is the asymptotic freedom that sub particles have in confinement, suggesting that the barrier is external rather than internal.
Space time is the candidate for a barrier. Maybe in this case the particle in the box example doesn't apply. A quantum particle cannot pass through the barrier and emerge from an opposite side, because space time doesn't have an opposite side.
A type of polarization in space time can be described for propagation of objects, for which the fractional charge is not able to perform the essential polarization. In this representation space time must undergo a quantum transformation for every object that passes through it. This is how the question fits into my larger interest.
The implication is that vacuum space might exchange energy and information with everything that passes through it.
The question is easier to answer with mesons because they are unstable and must decay. The fractional charges are not found in experiments.
CERN results are suggesting that vacuum space has a limitation of energy density which can be reached briefly in a small space.