BizTalk HttpServiceInstances not getting terminated

Execute the below query against BizTalk messagebox database.

Declare @out int
Begin transaction
exec dbo.int_AdminTerminateInstance_BizTalkHttpHost '3ae6ac2f-9b24-407a-a072-63733bc84254', '3ae6ac2f-9b24-407a-a072-63733bc84254', @out
Commit

where BizTalkHttpHost is the host name of the active instance
First GUID value is the service instance id and second GUID is service type id.

Remember this is provided as is with no gurantee.