I'm living on a campus near my university and everybody attending here can access the network... Since the campus is directly connected to the internal network of the university we get a 100MBit connection... The people elsewhere get much less (max is most likely to be 2MBit), therefor they think it's wise to use download resumers and programs like that. That by itself isn't a real problem, the problem is that,for some weird reason, they seem to think that opening one file 50 times on a server makes downloading it faster...
The question is now, how can I limit the maximum open files per user to something like 4 in Samba 3.0.13, ie not limiting the total amount of open files, just the amount per user...

I don't know if this would be obeyed by samba (it should in theory). There is a file on redhat systems called limits.conf

On redhat and Fedora core its located in /etc/security/limits.conf

In there you can set user specific limits for open files (both hard and soft limits). These limits are applied by a pam module so it will only work if your utlising that but in theory it should its a place to start looking.