Consider sending a reminder after some time period (1 day, 1 week, etc.) if they haven't reacted.
Optionally, remove users from the server after a certain time if they still haven't reacted (e.g. > 2weeks).
Low tech solution
Create a channel only visible to people who haven't reacted and occasionally send reminders in the channel.
Considerations
The goal is to just help people access the server - we want to avoid "shaming" them.