They can prevent the (further!) dissemination of the spec under copyright law (sfspec24.pdf is licenced for internal reproduction only, so e.g. I can’t just give you my copy), and they can prevent people from calling them like that under trademark law.
They cannot, however, prevent anyone from producing soundfonts or soundfont-related machinery, as this is required for interoperability. (Though without the availability of the spec this would become harder; the source code of OSS soundfont utilities like Polyphone and FluidSynth would become the de-facto standard, and if that deviates from the spec anywhere but the spec is hidden, it’ll eventually become the new de-facto spec, and commercial soundfont producers wouldn’t like that (because it’d mean their official-spec-compliant soundfonts won’t work identically in OSS applications) so they’d probably have pressure to not hide the spec.)
The spec is not distributable, but software and soundfonts implementing it are.
I’m also pretty sure that people may summarise the spec in their own words and distribute that, for interoperability, at least in the EU, but specific details may be tricky and they’d best consult their lawyers if they intend to do that.
IANAL, TINLA, TTBOMK
