How can you prevent a user from going forward, once an error in a field had been detected?

In my script below, if a “dash” is included in the item number I give an alert, but the user can simply hit return and continue.  Any way to return focus to the field in error?

Thanks for any suggestions or ideas!
Frank

function itemFieldChanged(type, name)
{
if ((name == 'itemid'))
{
if( nlapiGetFieldValue('itemid').indexOf('-') != -1 )
     {
       alert('Dashes Not Allow In Item Name Number');
       return;
     }
}