If you're unable to delete a lead from a list, it's because that list is currently assigned to an active campaign.
Prospi prevents deleting leads from lists that are in use to avoid disrupting ongoing campaigns.
How to fix it
Go to your campaign and remove the lead list from it (or pause/stop the campaign).
Once the list is no longer attached to an active campaign, you can delete individual leads freely.
Just want to stop emailing one person?
If you just want to stop emailing a specific lead without removing the entire list from the campaign, you can manually mark the lead as Do Not Contact instead. This stops emails to that person without affecting the rest of the list.