How to write a warm reminder to prevent bicycles from entering the stadium?

Writing reminders will not do anything. It treats the symptoms but not the root cause. As long as your entrance is not modified, those young people riding bicycles will still ride in when no one is watching.

The poster is worried that a cyclist will hit a runner and cause an accident (or an accident has already occurred), so there is no need to write it so tactfully, just "Bicycles are not allowed" in red letters on a white background, and then add Personnel on duty.

If there is no person on duty, it is easier to raise the height of the only entrance door. The higher the better (heightening is because some stadium gates are as high as road guardrails, and the height is not considered a door at all). Just make the door narrower and add curved doors in front and behind the small door. As shown in the picture below (the direction of travel for pedestrians is as shown by the arrow, and the narrow and winding passage is only large enough for one person to pass sideways)