When some item has some interface selected in its settings, then button "Remove" is grayed out and it's right.
But when interface used in the single Discovery rule, then button "Remove" is still wrongly available.

When you try to remove this interface - tremendous error appeared:

Error in query [DELETE FROM interface WHERE (interfaceid IN ('30279')) ] [Cannot delete or update a parent row: a foreign key constraint fails (`zabbix`.`items`, CONSTRAINT `c_items_4` FOREIGN KEY (`interfaceid`) REFERENCES `interface` (`interfaceid`))]
DBEXECUTE_ERROR [CHost.update -> CHostInterface.delete -> DB.delete]