The ability to search for exact phrases is one of the most requested features on many discussion boards. As suggested above, you can use Google search with the site parameter. However, doing so will find results in any subforum. If you're interested in just one subforum, make that subforum's name part of the search. For example, to find Ask a threads in OT, try this Google search:
"ask a" AND "off-topic" site:forums.civfanatics.com
Note the use of quote marks and the hyphen in the subforum title. This technique isn't perfect, but it's pretty good.