Fortunately, developers allow trackers and users to specify a flag to disable DHT. There are 2 parts needed to modified: Metadata and Tracker. Usually, metadata is uploaded to tracker to be available for other leechers so tracker is possible to modify the metadata.
The key flag for BitComet is "private". This flag is an integer value which must be set inside "info" key in metadata. For Azureus, key "azureus_properties" in top level metadata must be removed to disable DHT.