Limit apt repo to a certain arch?

From man sources.list:

The format for a sources.list entry using the deb and deb-src types is:

   deb [ options ] uri suite [component1] [component2] [...]
... 
options is always optional and needs to be surrounded by square
brackets. It can consist of multiple settings in the form
setting=value. Multiple settings are separated by spaces. The following
settings are supported by APT (note however that unsupported settings
will be ignored silently):

·   arch=arch1,arch2,...  can be used to specify for which
    architectures information should be downloaded. If this option is
    not set all architectures defined by the APT::Architectures option
    will be downloaded. 

·   arch+=arch1,arch2,...  and arch-=arch1,arch2,...  which can be used
    to add/remove architectures from the set which will be downloaded.       
...
The first line gets package information for the architectures in
APT::Architectures while the second always retrieves amd64 and armel.

       deb http://ftp.debian.org/debian wheezy main
       deb [ arch=amd64,armel ] http://ftp.debian.org/debian wheezy main

So try something like:

deb [ arch=armhf ] http://mirrordirector.raspbian.org/raspbian/ wheezy main contrib non-free rpi armhf