Could be a simple fix - How to stop drawing at the edge of the Canvas?!

Good evening everyone, 

This could be a really simple fix but I cannot seem to find an answer. I've watched 3-4 Youtube tutorials for Fresco and they've not helped! So my question is ...

How do I stop drawing at the edge of the canvas? 

In essence I would like to create a block/wall where I can draw up to the canvas and the brush stops, creating an edge which I can colour into? This is for general art and an NFT collection but I cannot seem to find the toggle/button/process. 

Any help is hugely appreciated!