Set Property Item Dynamically

I have  the following code in which is called in POST_QUERY trigger.

SET_BLOCK_PROPERTY( 'MY_BLOCK', INSERT_ALLOWED, PROPERTY_FALSE );
SET_BLOCK_PROPERTY( 'MY_BLOCK', UPDATE_ALLOWED, PROPERTY_FALSE );
SET_BLOCK_PROPERTY( 'MY_BLOCK', DELETE_ALLOWED, PROPERTY_FALSE );

The above does what I intend to do.

I have an item in MY_BLOCK which I would like to set true for insert, update and delete the above code.

I have tried as

SET_ITEM_PROPERTY('MY_ITEM',INSERT_ALLOWED,PROPERTY_FALSE);

However it didn't work the way I expected. How can I achieve this?

Regards