Technically, you wouldn't be blocking someone if you weren't Snapchat friends with them already, but there are two simple things you can do to prevent someone you're not already friends with on Snapchat from contacting you or bothering you ever.
How to block someone you aren't already friends with on Snapchat
So the two things you need to do NOW are:
1. Set your profile to private by only allowing friends to send you snaps. You can do this through the settings menu (tap on the 'cog' icon).
(there's more info on Snapchat privacy settings here: https://support.snapchat.com/a/privacy-settings)
2. Don't decline their friend request if they send one, just leave it hanging. While a friend request is still pending on Snapchat, the user cannot send you another one.
So they will have 100% no way of contacting you via Snapchat now. Even better than blocked!