Subject How to cill attachment from DB side

I am running Firebird 2.1 and I am receiving deadlock error with the transaction Id. From this Id I can find the attachment with the SQL:
select a.*
  from mon$transactions t
    left join mon$attachments a on (t.mon$attachment_id=a.mon$attachment_id)
  where t.mon$transactionid=2264690

From the attachment I can trace the computer and username but user is saying that she has no open program. So - the program is ended but the attachment is still registered in the server. The question is - how to kill this attachment with some SQL command or some gsec, isql, ... command?