How to Disable Hyperthreading?

If there’s no option in the BIOS — which is surprising, since I’ve always seen that option on Dell systems with Hyperthreading CPUs — then unfortunately you can’t disable it. For what it’s worth, as of this writing, the vulnerability you’re referring to hasn’t been turned into an exploit observed in the wild yet, in fact to my knowledge none of the variations of the Spectre and Meltdown vulnerabilities have been weaponized into exploits yet. Thus far all of these vulnerabilities have been demonstrated as proof of concept — which is a bit surprising considering how long ago some of those variants were reported and how quickly other vulnerabilities get exploits developed for them.