Additional notes: Currently attempting to remove an interaction model is resulting in a 204 rather than a 409. In other words if after creating a non-rdf resource you attempt to remove it by means of this call:
curl -v -X PATCH -H "Content-Type: application/sparql-update" http://localhost:8080/rest/mytest/fcr:metadata --data "PREFIX ldp: <http://www.w3.org/ns/ldp#>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
The result is 204 rather than 409.