Currently the only way to NOT accept a code suggestion / auto complete in the python window is to hit the right-arrow key. If you hit: escape, enter, or tab, none of these will "decline" the autosuggest. I would STRONGLY recommend that ENTER not accept the autocomplete.
Using ENTER to accept autocomplete means every time you want to run python from the window, if there is a suggestion at the end of your code, you have to FIRST hit the right-arrow key to decline the auto complete, THEN hit enter. ONLY TAB should accept autocomplete just like most other text editors and IDEs.