When you first set up your hosting account, they gave you some sort of login that's totally separate from your WordPress login: depending on where you're hosted, it might be called "CPanel", or it might just be in your welcome email. It should've been the first place you ever logged in, before you even installed WordPress.
Because the bad plugin you installed is locking you out of the back end, you'll need to go there to delete it. Whatever control panel your particular hosting company uses, it will have a way to view the files and folders in your account much like you would on your own computer. That's where you need to go.
If you can't find that information, you should contact the support team for your hosting account; it's nothing we can help you with here.