Hi Dave,
currently this is not possible (also there is a second table used directly in a php file so stripping db table wouldn't work).
The only way to simplify it a bit is to set a Default country (in backend JEM Settings - Global Parameters). So it will be preselected in venue form and there is no need to change it (but it's possible).
If this is not enough you could also use css rules to hide/remove the field, e.g.
Code:
div.jem_editvenue #jform_country-lbl,
div.jem_editvenue #jform_country {
display: none;
}