Someone may be reported for various reasons. Since Telegram isn’t designed for making friends, people will find it annoying when strangers or random people keep sending messages. It also applies to people who keep posting spam in Telegram group chats.
When someone is reported, Telegram’s moderator teams will receive and review the message. The moderators decide whether or not the message deserves this. If it’s spam or inappropriate, moderators will limit the account temporarily.
What happens to that account? Reported accounts can send messages only to people who save their numbers. They can also reply to any messages that come to them first. This will last for a few days so reported accounts must wait. But when it is repeated, the account may be blocked forever.