What could get an adman to walk away from $40 million in new business? Fear of the patent trolls.

That’s the derogatory term for people who buy up the rights to patents to make money litigating violations. While they’ve long been a headache of tech companies, recently they’ve trained their crosshairs on another sector: marketers and their ad agencies, which they go after for using such seemingly commonplace technologies as drop-down menus and placing product descriptions next to product images. Ad execs say the practice is costing the industry millions in settlements and licensing fees.

“The ultimate irony is that patent law was invented to foster innovation,” said Bradley Gross, general counsel for Society of Digital Agencies, a trade group for small- to medium-size shops that’s forging a plan to protect agencies from such lawsuits. “But what it’s turned into is a system where overbroad patents are turned into to swords, not shields, to rob the industry on what should be public-domain technology.”

The issue has gotten archrivals Google and Microsoft to jump into bed together. On March 1 the tech giants teamed up to file a legal complaint against one such company, Geotag. After Geotag sued more than 300 companies, primarily retailers and restaurants ranging from Applebee’s to Zale’s, because their store locators infringed on its “internet organizer for geographically or topically organized information” patent, Google and Microsoft stepped in to stop what it called the company’s “baseless” claims. Geotag and its lawyers did not respond to calls and emails requesting comment.

If Google and Microsoft can prove the patent is invalid in court, Geotag would no longer be able to sue retailers or restaurant chains for infringement. It would be one victory in an ongoing battle, but not all targets have Google- and Microsoft-sized legal and financial wherewithal in their corner.

There are more than 300 companies like Geotag, according to Mr. Gross. Geotag’s case isn’t the first patent infringement cases to ensnare marketers and their agencies, just the latest in a practice that’s snowballed over the past three years.

This is how it works: “Patent trolls,” legally known as non-practicing entities (NPEs), are companies that don’t sell products or services, but buy up patents from small inventors to make money off enforcing them. They cite an instance that could infringe their patent — like the store locator on Zale’s website — and write a letter to the company or file a complaint, most often in the Eastern District of Texas, a locale that often requires a big trip for execs who choose to defend themselves. “It’s a long drive from Dallas,” said Cynthia E. Kernick, partner at law firm Reed Smith. The district also has a reputation for favoring patent owners.