If you know this happens consistently for one attribute, you could take the dirty approach of downloading the XML as a string, using a regular expression to remove it, then passing to your import mapping. This is a fragile approach as it relies on the text being in the correct format, but could be a workaround.
As an example, this could be used to remove all “referentie” attributes.
replaceAll($XMLString, ' ?referentie=".*?"', '')