You cannot on many phones, and often phones will even lie when it says all categories are disabled, as some jurisdictions have laws against turning off some of the highest alert categories.
There was a tornado warning a few towns over a month ago. My spouse's phone for wireless emergency alerts kept going off every five minutes for the other town: no other warnings for our town specifically.
She ended up disabling the alerts entirely, which seemed a shame to me, but the ear splitting siren every 5minutes wasn't really conducive to our sanity or our dog's or our ability to actually hear the weather radio.
Moreover, it's a huge pain to get to the history of emergency alerts, which seems like a design flaw. Surprising how poorly a critical life system can be designed.
I've dealt with Android phones for which there was zero way to disable the highest alert, adb regardless. Unless you have root, it's often not possible.
Yes, GrapheneOS of course allows this.
Edit: re followup comment.
No, adb cannot be used in all cases to disable the packages involved. At all. Some phones refuse to lets users disable some packages, no matter what.
Yes, I know what I'm talking about.
As someone with decades of Linux and Android experience, who often works nights, having Quebec police use presidental alerts... CRTC regardless, to warn of a child abducted by a parent 2000km away from me, is an exceptionally strong motivator.
The sheer stupid of an alert you cannot control the volume, sound, and length of in any way, is absurd. Try going back to sleep after something screams at you, at your equiv of 2am is madness.
By god I hate those alerts.
So yes, I know. If you don't have root, and the phone won't let you disable some packages, you're done.
ADB can be used to disable it without GrapheneOS by disabling the packages providing the high level implementation of wireless emergency alerts. It's not something most people can be expected to do though. It's actually a lot easier to install GrapheneOS via the web installer than using the CLI ADB shell. ADB is also quite dangerous and people can be tricked into giving very invasive access to malware with it. We're not recommending that people use ADB for this but rather just noting it can be done.