Hi,
You would need to apply a service-policy to police the bandwidth to that user. As you will most likely be natting to the internet, you will need to apply this policy to the inside interface attaching to the user host. You would need to rely on windowing af the PC connection also to ensure the stream is actually slowed absolute. By this I mean that the download has already hit the ASA by the time we want to slow it down, so if its not a TCP connection, you may find that the ASA will need to buffer the rest of the exceeding traffic. Unlike routers, the ASA does not drop exceeds, it throttles.
access-list USER1 extended permit ip host 10.1.1.18 any
class-map USER1
match access-list USER1
policy-map Police-User1
class USER1
police output 3000000
Service-policy Police-User1 interface inside
HTH
Andy