How can I hide the resolution slider from the app menu?

Open Settings > Menu > Customize Menu Options > Customize App Menu Contents… and set Resolution to Hidden. This removes the resolution slider while leaving the other sliders independently configurable.

Choose When Expanded instead if you want to keep the slider behind the display block's more-options chevron. Choose Visible to put it back in the usual view. Changes are saved immediately. Choose Done to close the customization sheet.

The Display Mode submenu has its own visibility setting. Hiding the slider does not remove the available display modes or change the selected resolution, and you can leave Display Mode visible for occasional changes.

These are app-menu visibility preferences, not display-specific scaling restrictions. See Customize App Menu Contents for the other independently configurable items.