Is it permissible to curse devils, for example by saying: "May Allah curse your Iblis [devil]"?

It is permissible to specifically curse Iblis (Satan), as well as all other disbelievers. As for Muslims who commit major sins, it is not permissible to curse them individually, but rather by category, such as cursing the thief, the one who consumes usury, and the one who drinks alcohol.