There is little point in trying to push someone out of a market using multi frequencies.
Firstly, it is true that you will hurt them a little, but you will hurt yourself a lot more.
Secondly, if you do manage to push them off a route, they can just set up a route to a different airport, and return to the initial route once you have stopped the multi-frequency.
However, if you still want to push people off a route, you need to maximise the number of seats you dump on the route. You could do this by using a really big aircraft, or using multi-frequencies, or a bit of both.
If it is a short haul route, try to find a plane with a really low range, because they will be cheaper, then put on lots of multi-frequencies.