Mtoo, whenever you're signed into the site (as you were to post your question) click the big "My Photosynths" link at the top of the page. When you see the list of your synth cards, hover your mouse cursor over the upper right hand corner of any synth card until the cursor changes from the normal pointer to the pointing finger that is displayed when you hover over a link.
Click on this spot and you will be asked if you want to delete that synth and warned that there is no 'Undo'.
Normally there is an 'x' visible in this upper right corner, however it was accidentally masked when the site's design was changed on March 18. The Photosynth team is aware of the problem and is working on getting the 'x' visible again as soon as possible.