You really need to use a blowoff tube. Get some tubing from your LHBS and rig it up so that it blows into a bowl of StarSan. Otherwise you risk a big mess and possible contamination of your beer once all the sanitized liquid is blow out of the fermentation lock.
To rig up a simple blowoff tube you could use a piece of hard plastic through the cork, and then connect one end of your tubing to the plastic, and the other end in your bowl of StarSan. Make sure to sanitize everything, of course, before you hook it up. You're going to have quite a mess pretty soon if you don't.