The best way to compete against spamlines is to run a spamline.
since the only way to flood a route is to compete for green (full-fare) passengers, technically there is no such thing as a spamliner in AE. [see my 1st post in this topic for the details]
While that seems counter-intuitive, it is true - and something that one needs to wrap their mind around in order to play well. Pursuing a maximally-attainable valuation strategy is the only way to achieve the higher cumulative valuations needed to consistently attain valuation trophies in AE