There are differrent types leaf spots on maple caused by different fungi pathogens. By the symptoms you describe, it may be tar spot caused by Rhystima sp. or Phyllostica leaf spot. To be completely sure you need to compare closely the symptoms of your leaves with those on the references I am given you below. However, if other species of trees have the same spots, it will not be tar spot which is specific to maples. You may need to get a proper identification from you local agriculture extension service.
The best way to control the spots on your potted trees is to move them
in the dryest possible places and out of the rain and mists because the fungus needs lost of moisture to infect the leaves.
Fungicide treatment as recommended in your area may be needed if you cannot control the environment but are much easier to apply, and thus more effective, on small potted trees than big grounded trees where the sprays do not easily cover all parts of the leaves.
http://www.extension.umn.edu/projec...cnosemaple.html
http://www.gov.on.ca/OMAFRA/english...4/26hrt04a6.htm
http://plantclinic.cornell.edu/Fact...ot/tarspots.htm
Best of luck.