Quickly remove the end in your fermenter, clean it with something sanitized like a brush or paper clip, re-sanitize the tube and put it back in. During fermentation, there should be enough CO2 generated and pushed out of the opening to prevent O2 from getting in while you clean the blowoff tube.
What is the inner diameter of the tube? You should use at least 1/2" inner diameter to prevent clogs. Also, make sure the blowoff tube isn't jammed too far into the fermenter. The farther you push it in, the more likely the krausen will clog it.