No 'undo', but ... in the tool bar there is a 'arrow' (first pic) which is a selection tool. Click that first, then click on the errant stamp. A box should appear around that stamp and in the upper right hand corner of that box, a 'trash can' icon. (second pic) Click trash can. Stamp should be removed. (fingers crossed).
Also, to have the changes appear ... one needs the ability to run the site cron job ... there is a routine (now called 'task') in there that deals with updating that information:
Execute scheduled task: Prepare submissions for annotation (assignfeedback_editpdf\task\convert_submissions)
... started 08:48:39. Current memory use 8.9MB.
Convert 1 submission attempt(s) for assignment 1
... used 25 dbqueries
... used 0.011998176574707 seconds
Scheduled task complete: Prepare submissions for annotation (assignfeedback_editpdf\task\convert_submissions)
Guess those that have used PDF annotation tools before don't have any issues with the tool bar icons. Would be nice if a 'tooltip' box would appear to give a hint about each button in the tool bar. ;) Still, spending some tinker time with new tools like Annotate is well worth the time spent in the long run! ;)
'spirit of sharing', Ken