How do the following actions affect my website's 'Search Engine Optimization'
Pruning, locking, hiding forums. Let us say I heavily pruned some forums or even locked or made some of my forums invisible.
Would such actions have an adverse effect on my websites SEO?
To expand on what /a3 said, pruning and hiding forums will cause content to vanish, so I also assume that could affect SEO.
However, locking shouldn't have any affect -- unless search engines notice that nothing changes on that link and stop visiting as often. That said, my forum doesn't change much but I see lots of search bots visiting.