Started happening the other day as well for me. And only on this site. Probably one of their CDNs got hit through a vulnerability (saw a story on ArsTechnica yesterday that a CDN that is used by some major Netherlands sites was affected, so, could see others or MacRumors using same).
And, yes, try clearing data/cache/history. Think that solved my issue.
ADD: spoke too soon. Just got a spam popup on MacRumors, so, 99% sure some issue with a CDN and or MR site, in my case.
ADD2: if OP is not using a device that support ad blockers (eg. iPad3) in Safari, might try using a browser that does have built-in ad blocking (AdBlocker Plus for example). Seems to be working ok for me and MacRumors on my iPad (all other devices have ad blocking, so, not seeing the spam ad popups on anything else or any other site).