What's your actual success rate on reporting megathreads, do mods lock them on the regular basis? I am asking because I don't remember ever reporting one for that.
Let's say that I had a very good share of locked threads in relation to the number of reports, certainly at least 90% - although not so long ago there was a case where a user asked for an explanation as to why his topic was locked, and according to everything that others have written, it seems that regardless of the fact that the mods in most cases agree with the report, it is not something that is considered "good work" by many forum users.
Lately, that's why I don't touch such spam megathreads - people just love spam, and hate those who prevent it.