I've made the same stupid mistake a few years ago. Use some paper towel, one end crumpled and shaped into a thin cone, put it in the valve as deep as it can go (use a skewer to push it down to the core). Now let the board sit for a few days in a very hot place, in the sun, heated garage, etc. Check the paper towel now and then, it will be wet inside the valve, when it's dry you're done.
There are some methods that include rotation (centrifugal force) and drilling to dry the core, but I've found the above to work well (thanks Roger).