To prevent the tree from dripping sap, you must determine why the tree is "bleeding". Have you pruned the tree recently, or was there damage to the branches from a storm?
Your silver maple should be pruned when the tree is in a dormant phase (usually late fall). If damage has been done to the tree you will need to 'clean up' the affected area via correct pruning and possibly apply a dressing.

Google 'prune silver maple' for good info on proper pruning techniques and treating 'wounds' to trees.

A healthy tree should not be dripping sap. Check for evidence of disease, such as canker; or insect infestation, ie borers.

Call your local county extension service and you may be able to get a master gardener to take a look at your tree for free. Be sure and ask for a master gardener with an interest in trees.

ps- this is the time of year sugar maples are creating sugars for sap production- tapping takes place in the winter.

Good Luck!