Affiliation cookie

For some reasons ... i want the affiliation process to work only once ... as a 'parrainge' thing see somewhere else in the forum ......
I thought that the easiest way to achieve that was to check with a plugin if the customer previously ordered something or not and delete the cookie if I need to ...
setcookie("hikashop_affiliate", "", -3600);
But I've got issues deleting the cookie ... any reasons for that ? something I'm doing wrong ?

Thanks nicolas ... that's working fine ...