Can I restrict download speed of a particular device on a network?

Well, the answer depends on a couple things:
1) if your router has any kind of QoS or bandwidth tools. Most products don't, unless you have an alternative firmware. What kind of router do you have?
2) What kind of network traffic is on each machine? Is one machine just internet stuff, and the other doing file transfers or P2P?

Typically, routers don't let you limit the bandwidth on clients, they only let you prioritize. This is done through QoS, but in my experience, it doesn't really work for most things. For instance, I had a similar setup as you, one machine (roommate) was torrenting heavily, and I just wanted to browse pages. I set my QoS to be first in line always (set by MAC address) and his to be last in line. This only slightly helped. The reason I think is due to the amount of socket connections going on at once.