Microsoft purposely prevents you from doing this. The whole concept of the Event Viewer is to present to you certain events that may require your attention. If one could go in and delete any random event, then the system could – in a sense – be compromised without you knowing, therefore making it unsafe.
If you have an error event logged, find out what is causing the problem and fix it. You don’t want to patch a hole in a dam by sticking a wad of gum in the hole.
If something is logging informational or caution events too often, then many times the event log source (either Microsoft or a third-party) has some setting that indicates how often or to what level of logging is configured for the application. That is where you go to minimize the logging, not by doing surgery on the event log.