Did you try resetting your browsers?
Any recently added browser extensions or suspicious items in start-up that you need to remove or disable?
Did you seek assistance from your antivirus provider (AVG, McAfee, Norton, etc)?
Recommend you run Adwcleaner. https://www.malwarebytes.com/adwcleaner/
If the issue remains see this free removal guide from Malwaretips.com: https://malwaretips.com/blogs/remove-adware-popup-ads/ You may not need Hitman Pro (available as a 30 day trial if needed) but as mentioned above please note the recommendation to reset your browsers.
Other free scanners (if needed) that may help with this issue are listed in List of Malware Removal Tools
Regards…
http://blog.emsisoft.com/2015/01/27/top-10-ways-pups-sneak-onto-your-computer-and-how-to-avoid-them/