Is it possible to do a u-turn after entering the Ambassador Bridge from the US to avoid leaving the US?

While @Alex's answer confirms that it is possible, evidence from Street View suggests it might not be a good idea - doing this (especially for sightseeing purposes) is clearly bending the rules and could conceivably get you yelled at if the border guards are having a bad day.

Here are the three approaches to the bridge from the US side:

• Street View of exit (47B from I-75) clearly showing a sign saying "Bridge to Canada: NO RE-ENTRY TO USA" ...

• Similar picture for exit 192B from I-96):

• Southbound on I-75, it looks like you're going to be able to do it (no "no re-entry" sign!) (This is nearly the same location as the I-96/192B picture, but with a wider shot ...) This is the exit that the original post above references:

Coming in on I-75 from the North [i.e., southbound] you need to follow direction signs at Exit 47 C ,peel off on the right and follow the lanes into the customs compound merged with traffic from the south.

... but if you follow this exit you get to this fork:

where the right fork is "no re-entry" and the left fork (47C) appears to take you back on to I-75 going the other way ... maybe the rules were looser in 2014 when the original post was written ... ? The border guards might indeed let you make a U-turn to get back into the US (they're not bound by what the highway signs say), but it would be inadvisable to try it unless you have a backup plan. (For the record, this article referenced in comments points out that (as of 2014) there was a bridge employee whose job was to facilitate the return of people who were stuck in the restricted area (e.g. unable to enter Canada because of insufficient documentation or carrying restricted items) get back on the highway on the US side.