This is probably better handled on the computer (or user) in question, than on the router.
As the other two answers so far mentioned QoS, I'll point out that this will only be effective if the problem client is, say downloading over a protocol the rest aren't, or you want to limit ALL downloading on a certain protocol, or prioritize other types of traffic.
If, for example, this one computer is causing problems by downloading a bunch of YouTube videos, using QoS to throttle it at the router would also throttle regular web traffic, which might not be desirable.
In the past, I've dealt with this kind of problem in a number of ways, the best of which will depend on your environment. QoS is an option, though it's kind of a blunter instrument than you may want, as noted above.
You might also consider setting up a 2nd bandwidth-limited VLAN, if your router supports that - I've had to to do this in situations where a large number of users' personal web activity on the corporate network were generating too much traffic for our poor pipes.
You may also (at the router level) be able to apply MAC-address filtering so only authorized devices can get on the network (or so that specific devices cannot). Whether this is an option depends a lot on the router as well, and I'm not familiar with that particular brand.
Similar solutions might include client-specific settings at the firewall (if you have one), or fun with routing tables... such as redirecting all traffic between that client and YouTube to nowhere, or just all traffic from YouTube in general to nowhere (again, in the example that YouTube downloads are throttling your connection).
Alternately, as suggested by Andres, you can install bandwidth-limiting programs on that PC, and depending on the OS and environment you may be able to apply bandwidth limiting settings natively. If it's a specific user causing this disruption, consider using (or creating) acceptable network use policies to deal with it that way.
Finally, as it's a wireless router (and I assume the client at issue is connected wirelessly), have you considered trying to improve the performance over wireless? Are you having channel interference or wireless congestion, can you maybe improve the situation by adjusting the antennas or moving the router, or eliminating obstructions, etc? Looks like a consumer-grade wireless router, which would severely limit your options, in addition to the number of clients and amount of traffic it can handle without problems.