send an email to dept manager or your potnetial supervisor
about your application status directly.
我回信如下
Thanks for the offer. I am currently holding a job and my boss will keep me
as long as I wish to. In fact, my boss very much wishes me to stay. It is
really that the job is too simple for me, that I am looking for something
more challeging
Sorry that I could not accept your kind offer on contract position.
Appreciate that.
have a nice day !