There is a Help article on how to do this.
It says "Hover over DropBox icon and choose Ignore..." These instructions are incorrect
Misleading article: How to set a file or folder to be ignored - Dropbox Help
The instructions for Windows say Right Click & Choose "Don't Sync to DropBox.com",
The option is not present on the context menu (Win 10 & Win11)
The Help article should say
Use the PowerShell command. It's the only way to unsync a file or folder
Please fix the article
Spilly82