IF A GROUP of out-of-towners descended on a neighborhood bar and started trash-talking our sports teams, or Philadelphians in general, or our children in particular, we imagine that not many of them would leave without a few knuckle sandwiches . . . worse.

And yet, there's a group that's essentially been allowed to do that day-in, day-out, for years. And not only are they allowed to diss the city, but some of them are getting rich doing it.

These are the tax deadbeats that are taking all Philadelphians for a ride. They've bought up properties and walked away from them, thumbing their noses at paying property taxes. Some of them pay up only when they sell the properties. Others never do. According to a new report by Patrick Kerkstra for the website PlanPhilly and the Inquirer, these tax deadbeats have been allowed to depress the city's property-tax base by at least $9.5 billion. Those who own homes near delinquent properties are being robbed of property values by at least 22.8 percent.

The rest of us are being robbed because we're forced to pay higher taxes to make up the money that's not being collected.

Of the roughly 100,000 tax-delinquent properties in Philadelphia, at least 57,000 - 59 percent - are owned by investors, not occupants. Not all of them are from the suburbs, either.

These are staggering numbers, and they run counter to the stereotypical view of a tax delinquent as a low-income homeowner who can't afford to pay his or her tax bill. Only 21 percent fall into that category.

One reason that the city has been lax in going after tax deadbeats is that it bought into that stereotype. It didn't want to push people out of their homes by forcing tax sales of their property.

Clearly, the investors who are delinquents are playing the city. They know that local government is slower than slow in going after deadbeats. They can ignore the warning notices with impunity until the day comes when the property is developed or sold.

For every dollar in property taxes owed, Philadelphia collects only 84 cents. The national big-city average is 95 cents on the dollar.

According to the Kerkstra report, when it comes to tax delinquency, the Nutter administration talks big but carries a small stick. The administration declines to enforce a law that enables it to move quickly against delinquents, levying fines and placing liens on their properties, then moving them to sheriff sales. Nutter has vowed to spend $40 million over the next five years to develop the technology and hire the staff to go after delinquent accounts.

Eleven years ago, business leaders marched to City Hall in a "briefcase brigade" to say that they were fed up with the city's tax system. This led to reform, especially of the wage tax - a reform, incidentially, led by then-councilman Michael Nutter. The same thing needs to happen with homeowners: they need to descend on City Hall, in person or via phone or email, to let Council and the mayor know that we are tired of being bullied by slumlords who have systematically torn this city - and its value - down. Time for the legal equivalent of knuckle sandwiches for these bums.