On a "pre" event (like column Pre-Validate) you can reject the event (i.e. column value change being allowed) with a SetError.  There's no need to keep the previous value, as Sage will take care of that.

Some default settings will bypass such an event for fields like ShipVia, which is harder to deal with.

Ah, thanks Kevin!  

Just to leave a helpful info out here for others.   You made me realize I had to get a child handle for the criteria I was using.  The script was working well except the section that dealt with a GetChildHandle through ShipVia.   I had a handle of it, but I couldn't get the record back.  Evidently I needed to issue Find() to put it in the right record.   I find working with Child handle can be such a headache.  

Below works without following up with Find() after GetChildHandle

Set oCust = oBusObj.AsObject(oBusObj.GetChildHandle("CustomerNo"))
retval = oCust.GetValue("UDF_SomeUDF$",sVar)

But below doesn't work if i took out Find()

Set oShip = oBusObj.AsObject(oBusObj.GetChildHandle("ShipVia"))
retval = oShip.find(sShipVia)
retval = oShip.GetValue("UDF_SomeUDF$",sVar)

A ReadAdditional can help with that sometimes.

retVal = oBusObj.ReadAdditional("ShipToCode")

Set oShipTo = oSession.AsObject(oBusObj.GetChildHandle("ShipToCode"))