1
general::
Block the progress or completion of; also, intercept. For example, They worked round the clock to head off the flu epidemic, or Try to head him off before he gets home. [First half of 1800s] This expression gave rise to head someone off at the pass, which in Western films meant "to block someone at a mountain pass." It then became a general colloquialism for intercepting someone, as in Jim is going to the boss's office? let's head him off at the pass.
American Heritage Idioms
2
general::
Phrase(s): head someone or something off
Fig. to intercept and divert someone or something. • I think I can head her off before she reaches the police station. • I hope we can head off trouble. • We can head it off. Have no fear.
McGrawhill's American Idioms And Phrasal Verbs