They could but they'd have to retool the converted factory space back to making Falcons and there would certainly be disruptions.
They will not stop producing them until they are sure they can pull of BFR. The CTO has already said that Falcon 9 will be available as long as people ask for them.
By that point where they can no longer produce F9 they plan to have a fleet of Falcon 9 on the books that could continue for quite a while.