SQL Server : how to disable connections to a linked server?
I found the solution that I wanted:

-- how to disable a linked server

--check the existent linked servers
SELECT * FROM SYS.SERVERS
WHERE NAME = 'SWLON1'


-- Disable data access
EXEC sp_serveroption @server = 'SWLON1' 
      ,@optname =  'data access'     
      ,@optvalue =  'FALSE'

-- ABOUT procedure sp_serveroption
--https://msdn.microsoft.com/en-us/library/ms178532.aspx

-- the idea as how to disable a linked server
--http://www.sqlservercentral.com/Forums/Topic1038823-146-1.aspx