You are able to block numbers from leaving a message if you have a Pro subscription.
There are 2 ways to do this.
1. When viewing a message
2. Tap the more options icon
3. Select 'Block this caller' option
4. This contact will then display in the Blocked Callers list (in Menu -> Blocked Callers) and they will not be able to leave a message with HulloMail
1. Select Menu -> Blocked Callers
2. Tap on the '+' icon in the top right
3. Enter in a specific number in the field or simply select the contact you would like to block
4. This contact will then display in the Blocked Callers list and they will not be able to leave a message with HulloMail
Note: All illustrations are iPhone, please bear in mind it's the same for Android.