You'll have some sort of pop-up blocker on your Web Browser (or an 'Extension') or something that can be applied to it, that will allow you to block pop-ups for a particular site/URL ...
What web browser are you looking up the Forum with?
I am using Chrome on Win.10 and have the AdBlock working. I don't see any ads but I do see that pop-up. I have pop-ups turned on in Chrome settings because many sites now use a pop-up for their login, unfortunately.
You can do some customisation in the 'Options' menu off the AdBlock icon up at the top right of Chrome. There is a section in the Options menu called 'Customize' ... it's worth a look at and a play with as it appears that you can elect to have certain URL's add-free ...