* is not a valid option for user names, only IP addresses (and it has to be of the form 1.2.3.4, so 1.2.3.*, or 1.2.*.* or *.*.*.*, etc).
However, the ALLOW/DENY statements are parsed in a simple "first match" fashion, so you can obtain your desired configuration using
ALLOW_USER_ALL = user1
DENY_IPADDR_ALL = *.*.*.*
"user1" will be allowed by the first rule - and then the rule matching stops. Other users will hit the DENY line and no matter what they will be denied.