Here is the error i get after trying to DELETE the volume:
 
{
  "status": {
    "code": "SUCCESS"
  },
  "result": {
    "total_records": 1,
    "records": [
      {
        "key": "d9e35287-2f48-4fac-ad9f-b18c05757447",
        "parent_job_key": null,
        "owner": "api",
        "resource_name": "Volume",
        "status": "FAILED",
        "start_time": 1487934948061,
        "end_time": 1487934948237,
        "error_code": 4702,
        "error_message": "Volume voltestsm in Vserver ITNAPPOCSVMVM01 is the source endpoint of one or more SnapMirror relationships. Before you delete the volume, you must release the source information of the SnapMirror relationships using snapmirror release. To display the destinations to be used in the snapmirror release commands, use the snapmirror list-destinations -source-vserver ITNAPPOCSVMVM01 -source-volume voltestsm command. (errno=18436)",
        "request_method": "delete",
        "request_uri": "https://10.160.30.49:8443/api/2.0/ontap/volumes/f1d6062e-9374-40ed-97d7-63c0ad5ae012:type=volume,uuid=96a24105-2a82-43ab-ab7b-4c44700a6956"
      }
    ]
  }
} 
and none of the calls available seems to be able to RELEASE the relationship:
 
 
    get /ontap/snap-mirrors
        Retrieve all the snap-mirrors
    post /ontap/snap-mirrors
        Creates snap-mirrors
    get /ontap/snap-mirrors/{key}
        Retrieve the specified snap-mirror
    delete /ontap/snap-mirrors/{key}
        Deletes snap-mirrors
    put /ontap/snap-mirrors/{key}
        Modifies snap-mirrors
    get /ontap/snap-mirrors/{key}/events
        Retrieve all the events of specified snap-mirror
    post /ontap/snap-mirrors/{key}/jobs/restore
        SnapMirrorRestore job
    post /ontap/snap-mirrors/{key}/jobs/resume
        SnapMirrorResume job
    post /ontap/snap-mirrors/{key}/jobs/initialize
        SnapMirrorInitialize job
    post /ontap/snap-mirrors/{key}/jobs/quiesce
        SnapMirrorQuiesce job
    post /ontap/snap-mirrors/{key}/jobs/promote
        SnapMirrorPromote job
    post /ontap/snap-mirrors/{key}/jobs/resync
        SnapMirrorResync job
    post /ontap/snap-mirrors/{key}/jobs/update
        SnapMirrorUpdate job
    post /ontap/snap-mirrors/{key}/jobs/abort
        SnapMirrorAbort job
    post /ontap/snap-mirrors/{key}/jobs/break
        SnapMirrorBreak job