Need to stop hosting clients from traversing entrie drive VIA FTP

I am very new to managing a web-serverbut am doing my best to always persist

I am on a remote deciated server:
psa v8.2.0_build82070706.15 os_FedoraCore 6
Linux 2.6.18-1.2798.fc6
PLESK 8.2 running against FC6

Right now I have found that when a client is logged into his/her account VIA SFTP/SSH that they can go to not only any other clients server space but worse yet they can move through the entire server file system YIKES!…..

How would I stop this activity?

Thanking you in adavance for your help

Regards