Make it profitable. Pay for the work in BTC proceeds and use the strategy I suggested above. If we can show that attacks are profitable, then no one will start up these chains again.
The point should not be to double-spend or roll back txns. That is idiotic. It will not yield a positive return on investment.
Instead, you should merely seize all block reward from miners, enforce some tolerable fees on sends, and sell all altcoin revenue for BTC immediately. Kill the coins through taxation and dumping cheap coins on the market, not through vandalism.
People will either
1) weather the storm and accept a total loss (since the value of their holdings will be ~0 once the taxation is over)
2) Accept paying you part of their balance in order to get some money out. Cha-ching!
To make this work better, I would suggest gradually increasing mandatory fees over time. This should provoke a run for the exit. You don't want altcoin value to collapse too quickly. That is not profitable.
I wonder how much hash power a cartel would need to muster to be able to effectively pull this off on the current altcoins.
I wonder if the cryptocoin community in general would care enough to mount a resistance at this point and split away some of their bitcoin denominated cycles to fight an attack on an altcoin.
