You can hide whole sections, but not individual forums. On the forum home page, you'll notice little 'collapse' icons on the right side of sections. Use those to hide them. This may or may not be helpful, depending on what you want to hide.
Thank you Suzanne I asked this because I know that another forum that I am a member of has a feature that allows you to block out forums you don't go into so that the posts made under that forum don't appear under "new posts". I think their version of vbulletin might be different though