If you've played with KRunner, you probably have found the configuration where you can turn plugins on or off by pressing the left-most button with the icon of a wrench/spanner tool on it. This is something different, however.
Here is KRunner searching only for Applications on my machine:
Here it is looking only at command line operations:
You can do that with any of the following plugins (by plugin ID) in 4.7: recentdocuments, org.kde.windowedwidgets, locations, kabccontacts, bookmarks, org.kde.activities, solid, webshortcuts, shell, services, PowerDevil, wikipedia, nepomuksearch, desktopsessions, windows. This is accomplished by having a Runner plugin which either can match whole search terms or which registers a default syntax using AbstractRunner::setDefaultSyntax(const RunnerSyntax &). As long as the Runner plugin matches one of those criteria, then this line in the .desktop file for the plugin makes the magic happen: X-Plasma-AdvertiseSingleRunnerQueryMode=true. Unfortunately, in previous releases only a handful of plugins were marked this way. I've fixed this for 4.7.0 however. :)
To trigger the "single plugin" behaviour, you can create a launcher on a panel or desktop that calls to KRunner via D-Bus like this: "qdbus org.kde.krunner /App displaySingleRunner nepomuksearch". To get a list of possible values for that last entry, try this from a konsole: "qdbus org.kde.krunner /App singleModeAdvertisedRunnerIds".
The other way, and perhaps even more convenient, is to open the global keyboard shortcuts panel (type "global key" in KRunner :) and go to the Run Command Interface selection in the drop down box at the top of the window:
Then you can assign a keyboard shortcut to your favorite plugin. This means that you can, for instance, turn Nepomuk searches off in the main KRunner config and only when you want to search the files on your disk press the key combo for that runner. The rest of the time, the Nepomuk (or whatever other plugin you turn off in the KRunner config) won't be queried for normal searches done via Alt+F2 or "Run Command" from a launcher.
Pretty sweet, huh? :)