I agree that it would be nice to restrict the timeline by preventing retweets. Unfortunately twitter does expose this functionality.
The whole crux of that page revolves around a feature from Twitter called "Widgets." This means that in order to customize what each of those timelines looks like and how they behave is through the settings Twitter has made available to us. I went back in and double checked the settings and the only one that comes close is preventing Replies from showing up in the timeline (which I have turned off so replies DO NOT show up in the timeline). And even this setting is only available on the @cunycommons timeline widget. It is not available on hashtag searches (#cuny) or lists.
You can look at all the widgets I am using on that page by logging into our Twitter account, clicking on the gear icon in the right hand side of their toolbar and selecting settings. On the page that loads there is a sidebar on the left, click on Widgets and you'll see all the ones I've created during the development of this page.
I even went back to the Twitter developer page for this feature (their dev docs call it "Embedded Timelines" not widgets), and ran a find (cmd-F) on the page for "retweets" and it's only mentioned a few times - once to specifically state that retweets will be included in timelines like our @cunycommons widget.
I've added Boone as a watcher here because I know he just did some work with this feature on the profiles page. Maybe he's come across a hidden or undocumented setting or HTML5 data tag that could help us here.
But my answer right now is that this is just not a change that can be made. Not until Twitter makes it possible. We could send them an email asking for the functionality though. Let me know if that's something you're interested in and I'll draft up something to request this functionality.