This is slightly crude but could be sufficient enough for your needs.
I would try getting the resizeObject for your cropper and then locking
the dd element for all the handles.

e.g.

var resizer = cropper.getResizeObject();
// lock dd on all handles
resizer._dds.b.locked=true;
resizer._dds.bl.locked=true;
resizer._dds.br.locked=true;
resizer._dds.l.locked=true;
resizer._dds.r.locked=true;
resizer._dds.t.locked=true;
resizer._dds.tl.locked=true;
resizer._dds.tr.locked=true;