> Is there an option to disable custom altogether?

No idea if I could answer your question, but what
do you mean by "disable custom"?  What do you mean
by "custom"?  What is it that you're really trying
to do or asking about?

Customize user options start with `custom'.
I see these, using completion:

custom-browse-only-groups
custom-browse-order-groups
custom-browse-sort-alphabetically 
custom-buffer-done-kill
custom-buffer-indent
custom-buffer-order-groups 
custom-buffer-sort-alphabetically
custom-buffer-style
custom-buffer-verbose-help 
custom-define-hook
custom-enabled-themes
custom-face-default-form 
custom-file
custom-group-tag-faces
custom-guess-doc-alist 
custom-guess-name-alist
custom-magic-show
custom-magic-show-button 
custom-magic-show-hidden
custom-menu-nesting
custom-menu-order-groups 
custom-menu-sort-alphabetically
custom-mode-hook
custom-raised-buttons 
custom-reset-button-menu
custom-safe-themes
custom-search-field 
custom-theme-allow-multiple-selections
custom-theme-directory
custom-unlispify-menu-entries 
custom-unlispify-remove-prefixes
custom-unlispify-tag-names
custom-variable-default-form 
customize-customized-ignore
customize-unsaved-confirm-exits-flag 
customize-unsaved-ignore