Clear custom user picker field

[Jonny]

Hi All,

  I have some javascript that clears a field value depending on another drop down selection. This appears to clear a single drop down list field by setting it back to 'None', but it doesn't work for custom user picker fields. Is there different code I need to use for this?

e.g. the following line of code clears my HOP Approval drop down list field. Sets it to 'None'

$(cfHOPApproval).val('None');

However, Project Manager and Deputy Project Manager are user picker fields, and the following code doesn't work. I assume there is some other way of clearing the field?
$(cfProjectManager).val ('');
$(cfDeputyProjectManager).val ('');

Thanks.