Disable summary in category view?

In Settings > Reading > For each article in a feed, show: Summary, this defines what will appear in your RSS feed, for example when you view your URL like this http://sample.com/feed/. It does not affect the category view.

The category view is determined by your theme. You might need to edit your theme files, probably category.php. You might want to replace the_content (http://codex.wordpress.org/Function_Reference/the_content) with the_excerpt (http://codex.wordpress.org/Function_Reference/the_excerpt).

Another option is to always manually insert the <!–more–> tag when writing a post. This will cause the_content to display the excerpt in category view.