I can assume that you're relatively new to these boards. This topic was covered just a few months ago on these very threads. I can imagine that it's possible that in a few months, another new poster will ask the same question, so it's ok. You get a free pass!
I first heard the "Cram it, Clown!" story back in 1969 while I was a college student in San Diego. The person who told it to me swore that he saw it just the past week. I found that fascinating since 1) Bozo the Clown was not on TV in San Diego; 2) Bozo the Clown was not on TV in L.A., whose TV signals reach down to SD; and 3) There was no home video or Superstations in those days, so seeing Bozo on TV anywhere on the west coast would have been pretty difficult, as in impossible.
That perpetual story has been around about as long as there's been a Bozo on TV, going back to at least the late 1950s.
No one seems to able to trace the story's beginning, but I can tell you who perpetuated the myth and kept it going: The man who was in charge of Bozo's merchandising and licensing rights, Larry Harmon. Harmon, a proven liar who passed away recently, told reporters, media students, any audience he addressed the story, and there are recorded instances where he changed dates and locations. Harmon also falsely claimed that he created Bozo. Totally not true, and he was called on it. He backed off, then claimed that he never claimed to have created Bozo. The man seemed to have issues getting his stories straight.
When I was first able to see the Chicago Superstation, WGN, around 1982, Bozo was on every morning. I learned that those shows had been pre-taped the previous weekend. They taped 5 shows every weekend. With the show having been previously recorded, I highly doubt that the "Cram it, Clown!" incident would have gotten by the producers, that is if it had really happened in Chicago.
To sum it all up, the story is a myth, urban legend, BS, totally bogus, and it never happened.