I think passkey pairing will work for what you are trying to accomplish (user of central will need to enter a key)
You can set this up with the Hello_Client and Hello_Sensor samples apps from the SDK.
// Uncomment the following line to enable passkey pairing. Change passKey below to the desired key.
// #define PASSKEY_PAIRING
Using a whitelist may be another option, but you will need to know in advance the BD_ADDRs of the centrals: Adding and removing master nodes to a whitelist running on a peripheral device
Directed Advertisements could be another option: Directed Advertising
OK, thanks a lot.