While it is possible to skip fields for matching, it is not possible to select (or unselect) columns which should not be imported at all.
Right now, if columns are not matched, new properties are created instead - this is UX-wise not 100% clear during upload and this creates wrong data (bogus properties) when ignored.
I understand I should upload a CVS with only the columns I want imported, but I'd also like the option to select only the columns I want imported after the first step of uploading it.