Hide "MENU" Button on Mobile Devices in WordPress

You'll need to introduce @media queries for it.

Something like:

@media only screen 
    and (max-device-width: 640px), 
    only screen and (max-device-width: 667px), 
    only screen and (max-width: 480px)
{ 
.main-navigation ul.nav-menu.toggled-on, .menu-toggle {display: none;}
}

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(This example deals with iPhone 6 and iPhone6+ you'll need to sort it for all the devices to which you wish to serve content.)

Check out this link for an excellent break down of available common media queries:
https://responsivedesign.is/develop/browser-feature-support/media-queries-for-common-device-breakpoints/

Chris ~  Thank you for your reply.  I find your recommendation DOES work to hide the "MENU" button on the iPhone I tried, but not on an Android (as you mentioned to be the case).
I did review the link you provided, and checked out their reference to "Why you don’t need device specific breakpoints" (https://responsivedesign.is/articles/why-you-dont-need-device-specific-breakpoints/)

I would like to provision this site to hide the "MENU" button on ALL DEVICES (as it does not appear on a desktop or laptop, for instance).

How can I hide the "MENU" button on ALL devices?

Thank you! ~  Jacob

Intended to close topic by accepting Chris's excellent suggestion above; inadvertently accepted my own remark....

Please advise.

C Wah!  That works beautifully, Chris!!!  I added your second suggestion:

.main-navigation ul.nav-menu.toggled-on, .menu-toggle {display: none !important;}

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to the CHILD Theme (style.css), so, shouldn't create a problem.  It tests out perfectly on all devices I've checked.

Many Thanks! ~ Jacob

Great!!!!   Many Thanks ~ Jacob

You're welcome! As I said, typically it would be better to find the original css entries and edit those, but the !important feature does allow an override, just not the best practice. :)