There is no rule to use von. The genitive works with all names, such as von does with the Dative. It's your choice whether you want to use von or the genitive (von is a bit more colloquial, while the genitive might sound posh in places. Using von with a proper name that carries an article, see further down).
der Präsident der Niederlande
der König von Spanien
Spaniens König
Berlins Regierender Bürgermeister
der Schweizer Bundespräsident
der Bundespräsident der Schweiz
Proper names of geographic entities that are used without article typically get an -s appended in the genitive case. When names that are normally used without article carry one, the -s ending is often omitted:
die Zentralbank Europas
vs
die Zentralbank des geeinten Europa(s)
Proper names of geographic locations that only occur in plural, e.g. die Niederlande, die Vereinigten Staaten, always carry an article.
While not wrong, using von with geographical names that carry an article might sound uncommon, or even clumsy - native speakers will avoid this:
der Präsident von den Vereinigten Staaten
der Bundespräsident von der Schweiz