A crackdown is a strong action taken by authorities to stop a particular action. The verbal expression is to crack down on. (Ex. San Francisco is cracking down on tax cheaters.)
A tax dodger is someone who illegally avoids paying taxes; in other words, he or she dodges taxes!
A variation of this term is a fare dodger, who is someone who tries to ride the bus, metro or tram for free without buying a ticket.
Besides meaning ‘rigid,’ stiff also means more severe or difficult than usual. Tax dodgers in San Francisco will face stiff fines; the city is making the penalty more expensive. In another example of the term, President Obama is facing stiff opposition to his health care reform.