Do not create new nodes, only update existing I have a CSV feed that contains only prices and stock levels. The feed has to update only stock and price of existing products. What is the easiest way to block (just "in case") creating new products for the given feed?
First you need to ensure that each created node has a unique id associated with it that Feeds can use to identify it (eg as an existing node that needs updating). 

You can set one (or more) of the fields to be unique in the node processor mapping section (admin/structure/feeds//mapping):



My example takes the generic 'Item GUID', I would suggest that you might instead map SKU to GUID and then set that to unique (make sure to click both update and save). 

You then change the node processor settings to Update/Replace: