Microsoft Teams posts system messages to a team’s information pane to let people know about membership changes. You can’t stop Teams doing this because Microsoft doesn’t provide control over the system messages at the tenant or team level. You can obscure the names of new members by changing their display name, but maybe the best idea is not to add new members until the time is right.
Teams is removing system messages from the General channel in teams. System messages tell users about events like new members, but they’re really not that interesting, unless you’re fond of tracking the comings and goings of members, much like a noisy neighbor behind a curtain. Uncluttering the General channel is no reason to let it degrade, so keep it for important stuff and force conversations into other channels.