I'm afraid it's not possible to freeze completely a Forest Object if has references to custom objects, areas or surfaces, because these are needed to rebuild the object.
But there are several procedures you can use:
1) Convert to Tree Editor->Custom Mode
. In this case, areas are not longer needed, but you need to keep custom objects and surfaces, if used.
2) For small objects: Set Display->Viewport
mode to Mesh
and collapse to an editable mesh.
3) For medium size objects: convert Forest to stand-alone instances using Forest Tools
I hope that helps.