How can I prevent a plugin from updating unless it’s minimum PHP version is met?

At this time, this is not possible. Not without the PHP minimum version header being implemented.

When that is done, then it will be possible. Until then, you cannot DIY through plugin code alone.

In theory, you could release an update that would block future updates for that one plugin if the PHP version wasn’t met, but even then that would not prevent your plugin from receiving updates when it was not active. So, can’t be done because plugin updates are done by WordPress, not by the plugins themselves.