you could cap it with a handlebar plug if it fits, or a cork. there's a standard one on ebay right now that i might bid on. i was thinking i would use the part that holds the handlebars to mount a set of drop bars and use the original with the cowhorns that my wife uses. that way i could swap between bars easily for different riders. i already have two rear seatposts, one for my wife with her setup and one for everybody else so i don't mess up her setup as she is the stoker most of the time.