They are for advertising and information. You have shown an interest by "Liking" or adding them to your account so if enough people click on the links, somebody somewhere will get a penny or two.
If you hover over the link/advert top r/h corner, you might find a white x in a grey box appears. Click this and you will be asked to confirm deletion and you might even get a multi-choice survey as to why you are deleting the advert.
That might help stop them whilst you retain your interest in the group.