enhancing nokia application

I think there is already a protocol that support this feature but as far as I know, it hasn't yet been implemented. Remember that other than the technical issues (which aren't a major problem), there's also a legal issue here: are you allowed to delete a message that is stored on someone's mobile phone? Even if she hasn't read it, I believe she has the right to disable remote tampering with her device.
As for blocking sepcific numbers from calling or sending messages: again, it's technically feasible and I believe that some carriers enable you to do that.