The vaccines do reduce transmission in the minority of cases where full-blown symptoms last long enough that the reduction in infection length conferred by the vaccine is significant. But for most cases, they never progress to the point that the antibody response would become a factor, so most vaccinated people spread it just as much as if they were never vaccinated, in the early portion of their infection at minimum. So even if everyone got their booster today at the same instant, and every 6 months repeated that, it would still not stop the free movement of the virus among the population.
The only way to stop the spread is to do what China is doing, but with air-tight effectiveness, which is impossible.
That said, there's a big reason to get vaccinated and boosted anyway if you are in a risk group, since it is somewhat effective in protecting against serious and deadly outcomes.