Is there any possibility to limit the bandwidth of an SFTP user? My server has an upload of ~500kb/s and I don't want to 'spend' it all on one user
For openssh, see man sftp
-l limit Limits the used bandwidth, specified in Kbit/s.
If you are using WHM (Web Host Manager), you can simply go to "Limit Bandwidth Usage" under the "Account Functions" section. Choose an account and then enter a limit.
If not, you can use a utility called trickle - https://github.com/mariusae/trickle