I do not think you can detach a lov at run time.
I am not sure what you want to do, but I think there might be a work around.Can you use key_listval trigger on your text item to conditionly display or not display the LOV? like
if ...TRUE... then
Yes. you are right. I thought about set the lov_name to null but
I only looked the help says
"LOV_NAME Specify the VARCHAR2 name of an LOV to be associated with the given item. If the LOV name does not exist, you will get an error message. "
but did not do my own test.