Or is there an app that can do it?

I've tried the 'blacklist' option but it seems that people can find out that I've blacklisted them. Is there a more discreeet way where they can't find out?

Any help appreciated.

Cheers.

I've got an app that's called 'Firewall' which does what you say. Not sure if it's any good as don't use it.

I've been using the "black List" option on my SGS II and the "black listed" calls are forwarded automatically to my voice mail. Also, u might need to check this setting Applications-Settings-Call-Call Rejection- Auto Reject Mode = "Only black List".

I tested it myself and as far as the caller (the blacklisted one) is concerned there is no difference from my being out of service or turned off.

Extreme Call Blocker is the thing you're looking for.

Root Cal Blocker is so popular, don't know why! Maybe you could give it a try.
The built in blocker does the job very fine for me.

Regards.