String theory may not be proven but has the idea of treating the building blocks of matter as point particle been thoroughly refuted? For example Kaku writes:

You REALLY need to not use Kaku to learn physics. He used to be a serious physicist of the first order but his entire focus now seems to be on spouting nonsense to make money. There have been numerous threads on this forum blasting him, and here are a couple from elsewhere:

No. The infinities referred to in the quote were handled long before string theory was conceived, by means of the technique of "renormalization." The standard model is a theory of point particles, and is annoying successful in explaining the results of all particle physics experiments to date.

That said, you can view renormalization as saying that physics at the length scales we can probe experimentally is unaffected by whatever the actual physics is at much smaller length scales. That is, the "true structure" of an electron may be a point, or a string 10^-35 meters across, or a smiley face 10^-50 meters across, and we wouldn't be able to tell the difference: all these postulated structures are so small that they look like points to our experiments. Arguably, it would be more satisfying if the "true theory" consisted of objects of finite size, not points, so that calculations in the "true theory" could dispense with renormalization. But this seems like more an aesthetic principle than a refutation of theories of point particles.

I agree with this view of renormalization, however I'd say that a theory with finite size elementary particles instead of points would involve something more that just an aesthetic modification given the fact that in practice we can't "see" length much smaller than say 10^-22 m, if that was the case physicists would have dispensed with renormalization long ago.
The fact is that theoretically speaking the "point particle" premise is not dispensable because the SM and QFT and QM, and also classical field theory and mechanics are based mathematically on it by their intrinsic linearity, either in the non-relativistic galilean variety in the case of classical mechanics and NRQM or in the Lorentz invariance of the relativistic one in the QFT-SM case and relativistic formulations of classical field theory.