I tried using AudioSnap to tighten up some sloppy acoustic guitar strumming. Results were not usable so I gave up on it. But now a set of unequally-spaced vertical dotted lines is displayed over all of the tracks in a small section of the project. Presumably this is somehow AudioSnap-related. How do I get rid of it?

Select the clip, press Alt+A to open the audiosnap palette and disable it there.
 
Alternatively open the clip inspector tab and toward the bottom of that you'll see the "Audiosnap" section. You can also disable it there. 
 
HTH

FastBikerBoy
Select the clip, press Alt+A to open the audiosnap palette and disable it there.
 
Alternatively open the clip inspector tab and toward the bottom of that you'll see the "Audiosnap" section. You can also disable it there.

I think I did all that. I've even deleted the original clip that AudioSnap was applied to.
 
Unfortunately I'll be out all day today, so I won't get the chance to check until tomorrow.

AudioSnap appears to be disabled as far as I can tell. The vertical lines appear on all the tracks, within a 4-measure range, regardless of whether the track contains a clip there.

Sounds like you turned on the grid display.
However, make sure you don't have the track display set to "audio transients," it should be set to "clips."

FLZapped
...make sure you don't have the track display set to "audio transients," it should be set to "clips."

It's set to display Clips on all tracks.

When I opened the project today, I got a dialog box displaying the message "Transients are in the process of being generated...". This seems to imply that AudioSnap is still active somewhere in the project. How do I find out what exactly it is doing and where?

Pressing Ctrl+Alt+A which will select all audiosnap/stretched audio. It's also available in the Edit Menu--->Select...
 
If you don't get a selection that would tend to suggest that audiosnap isn't active. Having said that the selection method only selects clips that have had some sort of editing performed on them. I seem to recall this having come up before at some point, unfortunately can't remember the answer if there is one.
 
Another thought. Hidden tracks anywhere?

I discovered that the dotted lines disappear and re-appear when I close and open a particular folder in the project. By moving tracks one by one out of the folder, I discovered that there were 3 separate tracks causing the lines to appear. I then Bounced to Clips each of the short clips in these 3 tracks that fell withing the 4-measure range of the dotted lined. The lines seem to have gone permanently now.

It's still a mystery though, since AudioSnap was apparently not active.