1. A computer-implemented method for detecting advertisements displayed to users via user interfaces, at least a portion of the method being performed by a computing device comprising at least one processor, the method comprising: registering a program installed on the computing device as an accessibility service to enable the program to receive, via an accessibility application program interface provided by an operating system of the computing device, accessibility events designed to enable the accessibility service to assist impaired users when interacting with the computing device by describing changes in content of user interfaces of applications running on the computing device; after registering the program as the accessibility service, requesting to receive, via the accessibility application program interface, accessibility events in response to the changes in the content of the user interfaces of the applications that are likely to display advertisements; determining that at least one application running on the computing device is likely to display advertisements by determining that the application is known to host advertisements provided by third-party organizations; determining that an advertisement is being displayed to a user within a user interface of the application running on the computing device by determining that at least one accessibility event received by the program describes that the user interface is displaying content indicative of advertisements; and in response to determining that the advertisement is being displayed, performing at least one action to prevent the advertisement from interfering with interactions between the user and the application by preventing the advertisement from directing the user to content external to the application.