Ignore is specific to the member. They should not be able to start a new conversation with you if you have them on your ignore list.
Could you please confirm that they were on your ignore list before they started the new conversation with you?
Note: They will still be able to reply to existing conversations with you, but you can bulk Mute those conversations by searching for all conversations “Received by” the member, using the select all option, and then choosing Mute from the drop-down list of options.