It's impossible to take on more work at the moment.
The work can't be done at a moment's notice.
You must work more.
I'm up to the ears in work.
I can't feel up to this hard job.
No work can be done with ease.
This is a job that you don't have to do.
Work is preferable to idleness.
There is no one left who is willing to take this job on.
I don't want to take on any more work.