Probably better to do this within MC itself...
Create a smartlist (press F9) and in the wizard that pops up, press the import / export button and paste the following in there:
[Media Type]=[Audio] -[Stack Top]=-1 ~d=a [filename (path)]="J River Conversion Cache"
Press OK, twice, and the list of files presented to you will be all of the audio files in your conversion cache folder. To be certain, be sure you can see the "File Name" field in the list, scroll down the list and you should see that all the files listed reside in this cache folder. These files are all linked with with your lossless masters and MC uses them during a handheld sync for example, meaning that those tasks complete inordinately more quickly as you are not asking MC to carry out the conversion every time you sync.
If you really want rid, you can select every file in the list, right click, choose "Delete", and pause, take note the very important information below...
If you're tight for space, I'm guessing your recycle bin won't take 15 gigs, so you're going to want to delete from disk.
Please make absolutely sure that the "apply selection to all stack members" is NOT selected. If it is, you will permanently delete ALL of your linked audio files and I'm sure you don't want that!
Now that you've read and understood that, and checked your settings, you can press Delete.
This will empty your cache folder, and keep your stacks database in sync with the main library. You have now recovered your 15 gigs, but you're not quite done yet.
You should not need that smartlist again, so you can right click on it in the tree and delete it.
Next, you want to make sure that you have turned off the conversion cache so that you do not find yourself in this situation again over the coming months. To do that, open options (press Ctrl+O), and type cache in the search box.
Under "File Location" you will see "Cache Location" options for Audio, Video and Images. When you click there, you should see the three options, and the one you want is "None (do not create cache)"
Set those, and now you should be all sorted.
-marko.