The XML spec requires that parsers attempt to examine the doctype if it is
provided, since some information (eg entity declarations) _MUST_ be
considered part of the document's content. Turning off validation (which I
presume you've already done) doesn't change that.
So your only recourse is to make sure the attempt to retrieve the external
subset fails, and fails quickly. The standard kluge is to write and install
an Entity Resolver which recognizes attempts to fetch the external subset
and immediately returns empty/not-found, simulating a failed attempt to
retrieve the DTD file from a remote URI.
