If you are waiting out the 48-hour clock to re-block them, you can Mute them to stop seeing their activity in your feed.
If it has been more than 48 hours and you still cannot block the member, consider these other restrictions: If you are waiting out the 48-hour clock
LinkedIn enforces this "cooling-off" period to prevent users from abusing the feature—for example, by repeatedly blocking and unblocking a person to monitor their profile without appearing on their "Who's Viewed Your Profile" list. Quick Facts About Re-Blocking If you are waiting out the 48-hour clock