URL Parameters (sort/limit/order) - duplicate content?

Going over my site with GWT, got to the "URL Parameters" section, and there seems to be a problem.

Looks like Google is indexing every version of the category content, displayed in different order, sort limit, etc.

1.) What's the best way to handle this issue?
Add exclusions in robots.txt?
Use the Google Webmaster Tools to explicitly tell Googlebot not to index these URL's?

2.) Which of these should I exclude, and why? I'm worried about the message that this could cause a lot of pages to disappear from the index.

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By default opencart already provides support for this with help of canonical tags on category page.
So when you visit category page or search page with sort order, asc, limit, etc. Canonical tag is added to those pages and telling the search engine about actual page.

So the reason Google is indexing other pages is because you might not have this Canonical Tag.
I don't see any other reason for this besides this.

Hope this helps./