Hi, if you don't have Global contacts enabled (see Settings - On-line marketing - Contact management - Global data & merging), contacts are managed per site, so it should match exactly what you need.
If you have those "sites" implemented just as sub-folders of one logical site (Kentico doesn't know you have more sites), then it may be more challenging. In that case you might want to try to use contact groups to manage individual groups of contacts based on some specific custom flag, but it still won't provide that much luxury as the native per-site approach.