I am considering using a Spanned Disk as a
single logical drive is this advisable client server environment.
No. It's bit not advisable in a production setting. In a spanned drive setting, if a single drive failed, your system would become unusable.
Whats the best pratice for such a scenario.
There isn't You have not given enough info to give you any directed/specific advice. How are your drives configured now? Which drive is running out of space? Are you unable to free up space any other way? What kind of server?
A basic general best practice: use a quality RAID controller to create a redundant array out your multiple disks. This will likely require you to rebuild your server.