When I type an opening parenthesis, ENSIME automatically inserts a closing parenthesis. How do I disable this?
The completion in Emacs's scala-mode happens when you turn on scala-mode-feature-electric-mode (which is the "recommended" mode in the scala-mode documentation). If it's something that happens manually, look for the following lines in your emacs startup files and comment them out:
(add-hook 'scala-mode-hook
'(lambda ()
(scala-mode-feature-electric-mode)
))