Try this
- Put the definite answer before any warm acknowledgment. Remove maybe, probably and we will see if you have already decided not to attend.
- Keep an alternative invitation in a separate sentence only if you mean it. Review whether the message accidentally asks the organizer to keep a place open.
A worked example
Ambiguous: “It sounds lovely; I might struggle to make it.” Clear: “Thank you for asking. Please do not reserve a place for me; I won’t join this outing.”
An invented example to adapt, not a real exchange or a script you have to follow.
Before you leave it here
If the plan involves a ticket already bought, address that practical consequence separately using the actual booking terms. A clear decline does not by itself cancel a reservation.