I have ran into the same issue. We also have many jobs and jenkins slave nodes with limited space, and so deleting codebases at the end of jobs is necessary. Also for release jobs (of which we have many), I prefer to run a brand new force sync to retrieve the code to be built.
I am using the workspace-cleanup-plugin to delete job workspaces, however this is part of the solution, as it deletes the jenkins workspace, but does not safely cleanup the perforce workspace. It would be nice, if there was a post build action that could be activated to delete or at the least safely cleanup (i.e. revert any checked out files, delete any pending changelists, remove all workspace files) as a post build action that could be run before the workspace-cleanup-plugin deletes the jenkins workspace.
Unfortunately, clients need to be preserved in order for subsequent syncs to be more efficient. Deleting them at the end of the job would cause every run to sync everything from scratch, which is a massive waste of time and bandwidth. It's far more efficient to just keep them around.