How do I stop Notepad++ from showing autocomplete for all words in the file

Notepad++ provides 2 types of features:

• Auto-completion that read the open file and provide suggestion of words and/or functions within the file
• Suggestion with the arguments of functions (specific to the language)

Based on what you write, it seems what you want is auto-completion on function only + suggestion on arguments.

To do that, you just need to change a setting.

1. Go to Settings > Preferences... > Auto-completion
2. Check Enable Auto-completion on each input
3. Select Function completion and not Word completion
4. Check Function parameter hint on input (if you have this option)

On version 6.5.5 of Notepad++, I have this setting

Some documentation about auto-completion is available in Notepad++ Wiki.