yes, simply uncheck the box next to their name in the sharing page in the web app

if you uncheck the checkbox, they cannot write to the folder.

Okay, so I messed with this a bit more, and also found on another post that if you do not check the allow uploads option by someone's email than the files are read only and cannot be deleted. Maybe it's just my opinion, but it doesn't seem very intuitive...perhaps if it was a bit more clearly laid out by saying what the check box does to permissions (read/write etc.).