How are they creating these spam links on my forum?

[cbpayne]

I was just perusing the backlinks to my site and came across 1000's of spam links pointing to URL's on my forum. There were a lot of them.
This is one example of a page that some of the links were pointing to:
http://podiatryarena.com/proxy.php?....com/what-you-need-to-know-about-the-lottery/
Obviously something nefarious going on. I have not seen this before.

How can I stop this and get rid of them all?

(AHREF's backlink tool reports finding 79 of these spam...

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How are they creating these spam links on my forum?

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