There is no way to block IP's in DirectAdmin so you have to use solution like htaccess
I have included a rar file in this reply, unpack it then you see a file with no txt label, don't change it.
You can open the file with txt editor like wordpad.
Change the IP and save.
Upload the file in your public html dir. where your index file is located.
after upload it is normal that you can't see the file but it is there. You can see it in the filemanager from DirectAdmin.
If needed more help contact me by pm.