I know there hasn't been a reply to this for a long time, but I also wanted to disable the Document Map when exporting to Excel, and I discovered this solution.

In the DocumentMapLabel field for each object being mapped, use an expression that checks one of the "Built-in Fields" called RenderFormat.Name:

=IIf((Globals!RenderFormat.Name = "EXCEL") or (Globals!RenderFormat.Name = "EXCELOPENXML"), Nothing, "your DocumentMapLabel text goes here")

The formula checks the render format - if it is one of the Excel options, then it sets the DocumentMapLabel to nothing (so a Document Map won't get generated), otherwise it sets the DocumentMapLabel to whatever text you specify, and a Document Map will be created.