Is it off-topic? It's more on-topic than the posts you've highlighted.

Worse still, those posts often hang around for days, or even weeks.

I honestly can't believe it and I've raised it before.

The spam is deleted regularly. As you know we have a week-end as a days-off. On monday the forum is cleared of it. They never hangs there for week. 

In all honesty Ross, posts like those hang around long after you've responded to several on- topic questions. All forums I've been associated with have moderators who's sole duty is to keep the forum clean. I've been such a moderator on several forums and the rules were: "Deal with spam posts first and keep the forum clean".

Leaving posts like this is NOT good for the forum, the product or the moderators who can be easily accused for not doing their job.

It's pretty obvious that you have very little support from the other moderators and I fear where the forum would be if it wasn't for you. Oddly, moderators are not hard to find. Getting a really good one  like you is the hard part. However, answering questions and keeping the place clean should be two separate jobs.

Any chance you can discuss this with the other Mods and try to get this forum back on track?

Erik's screenshot should do the job.