There are a couple of things you can do to make it harder, but you can't stop people looking completely - to do that would mean that the code in the DLL wouldn't be readable by your application either!
Start by looking at
obfuscation[
^] - google can help you find obfuscators:
Google[
^]
But be aware that you can't completely protect any executable: if the processor can read it to run it, then so can a person! And it's very, very easy to put more work (and thus money) into protection than you would have saved in theft...