Wow, they changed channel. I'm not entirely sure why, but whatever. In that case, what you need to do is channel out the area, but one tile should be an up stair. Then you remove all the ramps, then the stair from above.

Just can't believe they changed channel.

Edit: Looks like they changed this too. Yeash. Well, just dig into the area from the side and wall it off then.

At this point I've resorted to creating an up ramp on the "outside" of my moats and leaving them dry. Tactically it makes more sense. Filling them with water makes them useless when winter sets in an ice forms, at least until I find magma, but having a moat that a dwarf won't die in when he falls in it is probably a good thing too.