Spam Detected! means you sent too many messages into one Inbox conversation too quickly, so Highrise paused your messages for a short time. The message shows a countdown and closes by itself when the time is up. It isn't a mute or a ban.
What should I do while the countdown runs?
Wait it out. The message can't be dismissed early, and it goes away on its own when the timer reaches zero. After that you can chat normally again.
The message that triggered the lock wasn't delivered, so resend anything important once the timer ends.
Why did it happen?
The limit applies to each conversation separately. Sending a long burst of messages, pasting the same text over and over, or firing off many short messages in a row into the same chat can all trigger it. Slow down and combine short messages into one.
Does this happen in Rooms too?
The Spam Detected! countdown is for Inbox conversations. Room chat has its own spam protection, and flooding a Room with repeated messages breaks the Community Guidelines. Room owners and moderators can also mute or remove players who flood their chat. See Why was I kicked, muted, or banned from a Room or World?
How is this different from a mute?
A mute is a moderation action. It shows a You Have Been Muted screen with a reason and an end time, and until it ends you can't use public chat, voice chat, private messages, or the feed. See Why is my account banned, suspended, muted, or warned?
It keeps happening even though I'm chatting normally
Contact Support through Settings > Support Center. Include your username, which conversation it happened in (a direct message or a group chat), roughly when, and a screenshot of the message.