There are two ways I see of going about this:
One is to call Airbnb and see what they can do to help. All their metrics and threatening e-mails are auto-generated. Once you explain that you're getting a slew of requests from the same party-going group they may help in some way and also call off the "decline too much" watchdogs.
Two is to modify settings & rules on your listing: I'm not sure if it's "discrimination" in Airbnb's eyes to require "Over 25" age in your house rules, but that's one thought.
I'd also add a house rule for "Only registered guests allowed on the property at any time". This, in conjunction with an exterior camera, can stop a party in its tracks.
Another is to switch on InstantBook and the Government ID requirement and positive host references, along with stricter house rules aimed at this group. If any reservation comes through that sounds fishy, call Airbnb to have it cancelled - it's much easier as an IB host "I'm not comfortable with these guests".