Others have embedded technology to block cheap refills.
They can pay to avoid them, use technology to block them or simply ignore them.
AT&T Wireless has developed a technology to block messages sent anonymously or to large blocks of customers.
The company was ordered to use filtering technology to block hate-promoting material from appearing on computers in France or face fines equivalent to $13,000 a day.
Mr. Nohl said he had advised the GSM Association and chip makers to use better filtering technology to block the kind of messages he had sent.
Similarly, the Ethiopian government received $1.5 billion in Chinese loans for training and technology to block objectionable Web sites, television and radio transmissions, according to exile groups.
Section 505 required that for a channel "primarily dedicated to sexually-oriented programming," a cable system that lacked the technology to block the signal completely had to limit the channel to hours when children were unlikely to be watching.
They have, as I have written previously for this site, been gradually cracking down on their citizens' Internet freedom using sophisticated technology to block access to certain content, and even arresting or physically harming people whose communications are deemed inappropriate.
A Silicon Valley start-up yesterday sued EarthLink, the big Internet service provider, saying that EarthLink's latest technology to block unwanted e-mail marketing, or spam, violates two of the start-up's patents.
The Iranian government, more than almost any other, censors what citizens can read online, using elaborate technology to block millions of Web sites offering news, commentary, videos, music and, until recently, Facebook and YouTube.
