The keyword is bandwidth management and the Linux Kernel has direct support of it. I dealed with this long time ago and have forgotten much about that but there is a great online book that covers the topic extensively. It's called
To get a foretaste of what you want to achieve take a look at
Perhaps there are (software) servers which support bandwidth control directly!?
Supplement:
In the pureftpd FAQ it is explained how to enable bandwidth limitation on the system level. On the first sight it seems to be applicable to other ftp servers as well.
there is another freely available book called
that deals with the topic. At first glance it seems to worth a look.