If the object in question doesn't have any kids, just @chmod object.prop !c
If the object has kids, there are several things that can help, but the easiest, and best way to handle it is to use a verb on $wiz_utils. Your fix would look something like the following:
;$wiz_utils:set_property_flags(#object, "property-name", "r")
Note that in the eval that you use, you DO want to have the name of the property and the permission bits you want, enclosed in double quotes as written above. For more info on this useful verb, type: 'help $wiz_utils:set_property_flags'.
(BTW, no, you don't need to be a wizard to use this!)