The Locator Bar is a compass-like bar above the hotbar that was added in the 1.21.6 "Chase the Skies" update. It shows the direction of other nearby players. Using the locatorBar game rule you can turn it off server-wide for all players.
Requirement
You need OP rights (at least level 2) to change game rules. See Grant OP Rights.
Disable the Locator Bar
You can run the command either directly in the in-game chat or via the server console in the dashboard:
/gamerule locatorBar false
Re-enable the Locator Bar
Set the game rule back to true:
/gamerule locatorBar true