You can only use the "-nr" parameter to prevent this, but you will end up with the top level artifacts only (no references).

The attachment is a "reference" and the "-i" works for the top level artifacts in most cases. For example, if the test plan includes a test case, and you use parameters "-a=testplan -i=testcase", the test case will still be copied. It is the same as your example. Similarly, the "-nr" parameter also prevents the associated test case from being copied.

The parameter "-is" does not work for attachments in this case either (works for test phases).

Since the source code is included in the package, you can find out all the logic by reading the code. Better yet, you can modify the code and build the utility the way you want. For example

        if (processor.isRecursive()) {
            Iterator<Testplan.Attachment> attachments = tp.getAttachment().iterator();
            while (attachments.hasNext()) {

The original code only checks whether recursive mode is on before getting the attachments, you can add code to check whether the attachment artifact type has been ignored (this logic needs to added to the handler class of each artifact type if that's what you want).