How to restrict each user to making only 1 comment per node? Does anybody know if there is any way I can restrict each site user to making only one comment per node on my Drupal 7 site? I haven't found anything yet.

I've just tried a complicated setup with flags and rules and triggers and actions and redirects. This didn't work too well.
Implement hook_form_alter() in a module. If the form is the node comment form, check if the currently logged-in user has commented on this node earlier. (You might have to write a custom function for this.)  If the user has commented on the node, disable the submit button and the textarea.