sober steve
A person who refuses to partake in any form of drug taking, in particular when drugs are easily available. 
It may also refer to a person who is under the influence of alcohol/weed and is acting silly,naive or immature. In this context it is used sarcasticly.
A person who is drinking an alcoholic beverage very slowly may also be called "Sober Steve".
 
Hey man why don't you stop being such a Sober Steve? I mean you can't come the whole way to amsterdam and not smoke some pot?