Norway Maple
Acer platanoides (GoBotany, Wikipedia, USDA) is an invasive tree which quickly replaces native trees. It sprouts leaves earlier than native maples and maintains them later in the fall. It is identified by having additional major leaf lobes (five instead of three) and its smooth bark. Another feature is that it has seed pods which are almost 160 degrees apart from one another (native maples have seed pods much closer together). The definitive feature is a milky white sap, whereas native maple trees have a clear sap.
You can check the sap by breaking off a leaf and watching the base of its stem to see if clear or milky white sap collects.
Removal
If small enough (small saplings), pull them out along with their root system.
If too large to pull out, they should be girded. Resist the urge to just cut down a Norway Maple, as the root system is persistent and will easily grow back. Even if girded, the tree should be checked in six months to ensure that it didn't regrow outer bark through the girded section. It will also sprout branches from below the girded section.
Common Mis-Identifications
Native Red and Sugar maples may be confused with Norway Maples. The definitive check to remove any doubt is to pull off a leaf, and watch the end of the petiole. If the sap that collects is white, it is a Norway Maple. If the sap is clear, it is a native Maple. If there is no sap, leave it alone and come back another time in the year to re-check.