Can't 'revoke' pensions - but 'fair value pension' should be computed. Instead of giving grandious bloated pensions negotiated by unions in bed with politicans - an affordable pension might be 1% of the base salary of the average of the highest 3 years worked, times the number of years worked. (This is the Federal FERS system since 1984).
But - consider their existing pension (in some areas, it might be 3% of their final year's salary INCLUDING Overtime times the number of years worked....so it could be 3 times more than what is 'affordable'. Tax the difference between existing pension and 'fair value' pension at 95%. [Per Supreme Court Chief Justice Roberts - the government can do almost anything if they call it a tax!! So while it might be illegal to reduce the pension amount you give them....you can sure use a tax to 'clawback' the excess amount that was negotiated in bad faith!]