This answer was written before you explained that the embarrassing results don't come from a list of recently used local files, but from the References scope.
It mainly targets at disabling local files/application history, but also disabling online search results from the Dash. Once you disabled the online results, you won't get those very ugly result thumbnails from Wikipedia any more.
I wrote a bug report/feature request about this issue against the
unity-scope-home on Launchpad to ask for a child protection filter that either entirely blocks NSFW content from the search results in the Dash, only blocks them if they are no exact matches of the query, or only blocks at least their preview thumbnail to make them less offensive.
If you agree with me that this would be useful and you have a Launchpad account (or don't mind creating one - it's free of course and you only need to enter an email address), please consider confirming this bug.
You do this by visiting the link below, logging in to Launchpad, and then clicking on the yellow pencil icon next to the line
This bug affects N persons. Does this bug affect you?
Select Yes and click on Change and you're done.
Here is the link to the bug report.
Old answer (disabling local file/application history and online search results):
To disable recording of your file and application usage that, open the System Settings and go to the Security & Privacy options.
Switch to the Files & Applications tab:
Here you should turn Record file and application usage OFF. Additionally you can/should also use the Clear Usage Data... button to get rid of old recordings.
Then you can also disable online search results (This is where the explicit images in your case were coming from!). To do that, switch to the Search tab.
Here you can simply turn Include online search results OFF.