I joined a month ago since when I am regularly getting adverts asking me to sign up for Giffgaff(!) This on Chrome on PC. Adverts come from Adclick and there is no option to opt out. Does not seem to be malware.
You can also use a small programme called ccleaner, the free version. You can use it to delete all types of rubbish including cookies. Watch when installing as you have to untick boxes or it will install other things you might not want.