echo 0 | tee /sys/devices/system/cpu/cpufreq/boost

Might work, might not, depending on unknown factors. Whatever, I just wanted to make some all-core-but-unimportant process to run without going 95°C. For that it worked perfectly well. CPU temps of a Ryzen 5 3600 after many hours of full utilization were at ~65°C. CPU frequencies were capped to 3.6GHz with this while jumping up to 4.2GHz (and ~94°C) without.

Obviously this has an impact on performance.

To re-enable just echo a 1 instead. This is reset anyways when you reboot your system.