How can I disable smart indentation in emacs (and force either spaces or tabs)?

The problem is that each mode in emacs defines the TAB key diferently. To get a global behavior, look at Trey Jackson's answer in https://stackoverflow.com/questions/344966/sane-tab-in-emacs

 (defvar just-tab-keymap (make-sparse-keymap) "Keymap for just-tab-mode")
 (define-minor-mode just-tab-mode
   "Just want the TAB key to be a TAB"
   :global t :lighter " TAB" :init-value 0 :keymap just-tab-keymap
   (define-key just-tab-keymap (kbd "TAB") 'indent-for-tab-command))

You may want to use 'self-insert-command instead of 'indent-for-tab-command as pointed out by another person answering the question.

Try this.

Find your .emacs and add this:

(setq c-basic-offset 2)

this make your emacs inserts 2 spaces, you can change the number and put 4,

(setq-default indent-tabs-mode nil)

if you want spaces NO tabs