My impulse, which I try to avoid but fail to do so in most situations, is to look away or around. However my speech therapist told me to try and hold their eyes when a blocking comes, because in my case (idk about yours) by keeping eye contact, the muscles on my neck are less tense and so the vocal chords are more relaxed, thus lessening the blockings. But still it's pretty hard to me to follow through when I'm nervous.