Assuming the leaf spots are caused by a phyllosticta, there isn't much
to worry about. If you want to do something, you could water your
tree without wetting the leaves, and get rid of fallen leaves. But if
your yard has a lot of maples in it, your bonsai are bound to get
infected.

Of course, there are more serious leaf diseases of maple, like
anthracnose, but even so, these are things only to worry about in very
wet years.

As always, a well-watered, well-fertilized tree can take care of
itself.

Nina