How to remove Italian browser ramsonware? Arma dei Carabinieri - Attenzione! Il Suo computer personale è stato bloccato per
Here is a link for removing ransomware on a Mac.
it's pretty easy:
http://www.thesafemac.com/fbi-ransomware-virus-rampant/
From the post:

There have been many reports among Mac users recently of being “infected” with an FBI ransomware “virus,” telling them that their computer has been seized and that they have to pay money to get it unlocked. Fortunately, from the reports I’ve been getting, there is no indication that this is actually Mac malware of any kind. It’s simply an obnoxious browser pop-up, displayed via JavaScript, in an attempt to fool people into paying.
I have yet to see this behavior first-hand, but it’s undoubtedly being caused by JavaScript that has been injected into legit sites, either through malicious ads or by hacking the site. Keep in mind that the old wisdom that you’re fine if you avoid “bad” sites is no longer particularly good. Most malware these days is distributed through legit sites, so any site could potentially fall victim.
force quit Safari
The question, then, is what to do if you see this pop-up. If you cannot close the window or the browser, then you will need to force the browser to quit. Press command-option-esc to display the force quit window. In that window, make sure your web browser is selected, then click the Force Quit button. Once the browser quits, you can close the force quit window.
Depending on your browser and/or system settings, your browser may try to reload the pages that were open the last time it quit. This could cause the problem to recur immediately, as soon as you open your browser again. In Safari, you can avoid this by holding down the shift key while opening the app.
Once you have your browser open again, you should be fine. You could clear your cookies and browser cache if you like, but that really won’t be particularly useful. Contrary to popular belief, this will not cure such problems or prevent them from occurring.