Emboldened by Rush Limbaugh’s public apology over the weekend to a law school student whom he had called a “slut” and a “prostitute,” critics of the radio talk show host are intensifying their online campaign against his advertisers.

The apology, they said, was a signal that the campaign was working. On Sunday, a seventh company, ProFlowers, said that it was suspending all of its advertising on “The Rush Limbaugh Show” despite his apologetic statement a day earlier.

For now, the ad boycott is uncomfortable but not crippling for Mr. Limbaugh, who is estimated to make $50 million a year and whose program is a profit center for Premiere Radio Networks, the company that syndicates it. The program makes money both through ads and through fees paid by local radio stations, and while it often has sparked outrage during more than two decades on the air, efforts at ad boycotts in the past have had no measurable effect. Liberal groups and activists, however, hope that this time is different.

Mr. Limbaugh has been roundly criticized for talking at length about the sex life of Sandra Fluke, a Georgetown University law student who testified in support of the Obama administration’s requirement that health insurance plans cover contraceptives for women. For three straight days he lambasted her, before saying in a statement Saturday afternoon that he did not intend to attack her personally. “I chose the wrong words in my analogy of the situation,” he said.