By default, new relationships are created as 1:N in Visio. Most of the time, this is exactly what you want.
If you need to convert a 1:N relationship to 1:1, then highlight the relationship (click it). This will display a set of properties about the relationship. Switch to the Miscellaneous category of properties, then change the cardinality to "Zero or One" or "Exactly One."
This will make the appropriate change to the symbols on the child-side of the relationship.
