First, all so-called "anti-virus" or "anti-malware" software is harmful in one way or another, is not needed, and should not be used.
Your problem is most likely not caused by malware. It's a Web scam that only affects your browser, and only temporarily. There are several ways to recover.
1. If you're running Safari 8.0.7 or later and you see a dialog with the message “Don’t show more alerts from this webpage,” check the box and click OK.
2. If no other tabs or windows are open, or if you don't mind losing those tabs or windows, force quit Safari. Relaunch it by holding down the shift key and clicking its icon in the Dock.
The following steps may enable you to keep the state of other open tabs and windows.
3. Press the key combination command-W to close the tab or window. A huge box will pop up. Press the return key and both the box and the page will close. If that doesn't happen, continue.
4. Press and hold command-W. You may hear repeating alert sounds. While holding the keys, click the OK button in the popup. A different popup may appear, which you can cancel out of as usual.
5. From the Safari menu bar, select
Safari ▹ Preferences... ▹ Security
and uncheck the box marked Enable JavaScript. If the Preferences menu item is grayed out, go to Step 2.
After closing the malicious page, select from the menu bar
Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data
to get rid of any cookies or other data left by the server. Open your Downloads folder and delete anything you don't recognize.
