Is it possible for people not on my network to use my pihole dns server for blocking and what not?
Yes, you can use a VPN tunnel to securely connect remote machines to to your local Pi-hole.
See e.g.
https://github.com/pi-hole/pi-hole/wiki/Pi-hole---OpenVPN-server
While using vpn would normally be a great way of doing this I'm not able to install a vpn on the remote router
Is there a way of doing it using my public static ip?