Google is your friend: Be nice and visit him often. He can answer questions a lot more quickly than posting them here...
A very quick search using your subject as the search term gave 80,000 results:
Google: "Stop DLL Injection"[
^]
The second hit looks very useful:
http://stackoverflow.com/questions/869320/how-would-i-go-about-prevent-dll-injection[
^] (as does the third, fifth, ...)
In future, please try to do at least basic research yourself, and not waste your time or ours.