Hi,
in case anyone is interested, I'm also from Johannesburg and had the exact same issue when trying to install the spam filter addon. Constant hash sum mismatches and apps refusing to install
We use an Axxess lite account, I connected to an mweb business ADSL account (I had the issue before and fixed at my clients site running on a Neotel Uncapped account) and ran the following:
apt-get clean
apt-get -f install
I did run a dpkg -configure -a earlier but I don't think it was necessary.
and let it run, it installed all the missing spam filter files/dependencies. Seems its an issue with some of the ISP's having corrupted copies of the app files on their proxies, I know quite a few of our ISP's do caching (Telkom, Axxess apparently and iBurst) So maybe try running a different ISP, neotel and Mweb business seem to work nicely.
So yeah dmorris, looks like option 3 it is