You have in form a trigger called OnAfterInput. In that trigger you have original value before auto complete. You could make check item there and later on VALIDATE trigger raise an error. It’s a possible idea.
I don’t remember if there is a property to avoid such behaviour.
In the “No.” field in the sales line, find the property “ValidateTableRelation” and set it to NO, then you may need to modify the OnValidate code to make the error messages work properly, since in sales line, a lot pf processing is done BEFORE the code does an Item.GET, so it can be confusing.