Here's what worked for me:

 1. use `diskutil list` to find the small Microsoft Reserved partition you're trying to delete but disk utility (or the command-line equivalent `diskutil`) won't allow.  Let's say it's on disk 99 partition 400 (mine was on disk 0 partition 4)
 2. `sudo diskutil eraseVolume JHFS+ deleteme /dev/disk99s400`

The above reformats the partition to journaled HFS+ and once you've done that you can just simply delete it (and then resize etc. as you wish).

*Note:  I've used /dev/disk99s400 because that should not ever work, you have to replace those with the right disk and partition.*

**WARNING: Destructive, high risk. ALWAYS do a full back up first before screwing with partitions.  ALWAYS.**