If a sign says "Swimming Prohibited," don't go for a dip. It's not allowed. To prohibit is to forbid, or to disallow something.
In school, you are prohibited from leaving the premises before the end of the school day. The school administration also prohibits smoking and chewing gum. In 1920, it became illegal to sell alcohol in the United States. This time period when alcohol was prohibited is known as the era of Prohibition. Famously Prohibition did not work. When you prohibit something, you often make it seem more appealing.