O Nginx usa URIs normalizados internamente:
The matching is performed against a normalized URI, after decoding the text encoded in the “%XX” form, resolving references to relative path components “.” and “..”, and possible compression of two or more adjacent slashes into a single slash.
Use aspas (simples ou dupla) para envolver o espaço em uma string.
Então seria:
rewrite "^/top100/Armin van Buuren/(.*)$" /armin-van-buuren/$1 redirect;