It isn't possible to completely delete a client from Collums, use the archive function instead.
Go to Admin>Clients
Scroll all the way to the right of the screen and click the Options button of your client
Select Archive
To see all of the clients on your system including those that have been archived, selected the Show archived/unverified checkbox in the top right of your screen.
Unarchive a client
Follow the instructions above but select Unarchive form the Options button menu.