You're using ModelCon.remove(s,p,o) which doesn't allow nulls. You want to use Model.removeAll(s,p,o) instead which does allow nulls and uses them as wildcards. Here are the corresponding Javadocs (with emphasis added):
ModelCon.remove
Model remove(Resource s, Property p, RDFNode o)
remove the statement (s, p, o) from this model and answer this model. None of s, p, o are permitted to be null: for wildcard removal, see removeAll.
Model.removeAll
Model removeAll(Resource s, Property p, RDFNode r)
Remove all the statements matching (s, p, o) from this model.