Over the last couple of days I've noticed a sudden jump in what seem to be bogus files with one specific file extension that have been showing up in my search results.

Is there a way to exclude search results for some specific file extension?

Yes

In LW 4: Tools > Options > Filters > Keywords. Add the file extension that you want to filter (eg .wma, .wmv, .mov etc) > apply > ok.

In LW 5: Tools > Options > Search > 'Filter file extensions' > add the file extension that you want to filter out > Add Extension > ok > click ok on the main Options page.

You might find this helpful: Keyword & extension filter words to Add

Thank you, birdy and LOTR.