If you are using Safari, it may be due to a cookie they've placed on the hard drive.
From your Safari menu bar click Safari > Preferences then select the Privacy tab then click: Remove All Website Data
Restart your Mac to test.
If the problem persists, download the (FREE) Malwarebytes malware removal tool.
If you are not comfortable downloading the removal tool, you can use the instructions for removing malware manually > Stop pop-up ads and adware in Safari - Apple Support
Hopefully that will eradicate the problem.
I noticed that your profile indicates your Mac is running v10.11.2. If that is the case, the best way to avoid malware is to keep your OS X software up to date at all times.
The v10.11.4 update is available from the App Store.
Another way to avoid malware is to make sure Gatekeeper is enabled.
Open System Preferences > Security & Privacy then select the General tab.
Select: Mac App Store and identified developers
If that area is grayed out, click the padlock icon to make changes. You may be prompted for your admin password before proceeding.
* message edited by CS
