moderated Re: If possible, make "disable other reply options" apply to email replies in all cases #suggestion
With the exception of the "reply only to sender" topic setting*, there is no way for groups.io to actually force a particular reply behavior on each and every group member.
The reply-to setting for a group or topic affects only the corresponding field in the outgoing message header (for individual messages), to wit:
This amounts to a suggestion, and that's all it can do. Whether the recipient complies with that suggestion depends on that person's own behavior (e.g.: reply vs reply all), settings and capabilities of his email provider, and especially the settings/behavior of his email client.
In particular, multiple email clients are known to fail to comply with RFCC 5322, at least by default. Instead of honoring the Reply-to: line, they send the message back to the From: address. Remedying this requires knowledge of both the provider and the client, and even then may not be fixable.
Hope this helps,
*"Reply only to sender" is different in that if the recipient attempts to send a message back into the same topic, groups.io actively blocks it.