as blklab said;or a water hose works great.I took a hose with a spray nossel aimed it at the pen the dog is in,tied it down where it can't move,and put a lever handle off and on switch(one that you just pull and it is fully open).when the dog barks I just pull the handle.the spray will get him and he doesn't see you;so he doesn't know you did it.you should tell him in a firm voice to "NO".you have to be consistant with it.
the collar is the only other way I know.