T-Mobile US is planning to take unlimited data plans away from customers who use workarounds to bypass limits on tethering.

Customers who buy "unlimited" high-speed data for smartphones are also given up to 7GB a month for tethering, which lets a phone act as a Wi-Fi hotspot for other devices such as laptops. But some customers are masking their tethering activity and using as much as 2TB in a month, T-Mobile says.

T-Mobile's CEO promised to "eliminat[e] anyone who abuses our network."

Customer experience is my top priority & that means eliminating anyone who abuses our network. #byebye http://t.co/VdfpTBIQ3J — John Legere (@JohnLegere) August 31, 2015

In a blog post, he wrote that T-Mobile will be "going after every thief, and I am starting with the 3,000 users who know exactly what they are doing." Those users will be contacted today. They won't necessarily be eliminated from T-Mobile's network, but will lose access to unlimited data plans if they don't change their ways.

In an FAQ, T-Mobile said it has "developed technology that can detect people who deliberately choose to break our terms and conditions." Initially, customers will be warned, but if they keep violating the conditions they will be moved to a limited plan. Those limited plans are automatically throttled once customers reach their high-speed data limits. The limited plans are cheaper if you use 5GB or less per month, but customers can pay extra if they choose to buy more high-speed data.

Tethering abuse

When unlimited data customers exceed 7GB of tethering data in a month, their tethering speeds slow down unless they buy more LTE tethering data, Legere's blog post said.

"However, these violators are going out of their way with all kinds of workarounds to steal more LTE tethered data," he wrote. "They’re downloading apps that hide their tether usage, rooting their phones, writing code to mask their activity, etc. They are 'hacking' the system to swipe high-speed tethered data. These aren't naive amateurs; they are clever hackers who are willfully stealing for their own selfish gain. It’s a small group—1/100 of a percent of our 59 million customers—but some of them are using as much as 2 terabytes (2,000GB!) of data in a month. I’m not sure what they are doing with it—stealing wireless access for their entire business, powering a small cloud service, providing broadband to a small city, mining for bitcoin—but I really don’t care!"

Legere wrote that these customers "could eventually have a negative effect on the experience of honest T-Mobile customers."

T-Mobile already had measures in place to prevent heavy users of unlimited data from impacting other customers. Unlimited data customers who use more than 21GB in a monthly billing cycle are throttled for the rest of the month "in times and at locations where there are competing customer demands for network resources," T-Mobile says. That policy applies regardless of whether the customer is tethering.

Theoretically, that means anyone using 2TB a month is already being slowed down in areas where there's network congestion. They can only keep using high-speed data when the network is running smoothly.

But due to technology limitations, T-Mobile might not throttle these users at cell sites that only recently become congested. T-Mobile determines whether a location is congested "based on network statistics for the most recent quarter," rather than in real time, the company says.