Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to ask friends not to tag you in posts, use this factual base: name the platform, content, frequency, audience, or account access that must change.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. State the requested removal or contact rule and use the platform control that supports it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not trade passwords, monitor accounts, or move to a new platform after someone declines contact. Report impersonation, intimate images, account compromise, threats, and doxxing through platform and specialist safety channels. You do not need the other person to agree with a boundary for you to follow it consistently.