Stop your server and Go to the dashboard of your server and scroll down to Resourcepacks section.
There you should see your resourcepack listed, tick it to off. Refresh the page and double-check it is still ticked to off. Start your sever, and it should be gone.
If it does not work, stop your server, go to your server.properties, remove your resourcepack from the resource-pack = line, save changes, and start your server.
If it STILL does not work, stop your server, upload a new resourcepack to the resourcepacks section, tick it to on, refresh the page to confirm, and start your server. Then go back and stop your server, tick it to off again and double-check it did stay to off, then start your server.