Is it forbidden to practice "practical" kabbalah (kabbalah maasit) today? If so, why? If it is forbidden to practice is it also forbidden to study?
The dispensation to practice Kabalah Maasis via Sefer Yetzira is codified in Shulchan Aruch (YD 179, based on Sanhedrin 57b). The Shach there (:18) quotes the Levush who states:
[The reason this is permitted is because] they are Holy Names, and Hashem gave them the ability to be able to perform things through the pious and prophets. One who utilizes them demonstrates the greatness and strength of Hashem. However he should make sure to delve into them in purity and holiness and for the sake of the Holiness of Hashem, or for the sake of a big mitzvah such as the honour of Shabbos, which we are not capable of doing in these generations, and even in the time of Yeshaya we find that he was punished for this. Certainly in our times it is not possible to act in purity and holiness and Hashem desires a [sincere] heart. This is the words of the Levush, and his words are correct. And similarly we find in several compilations that one should only use the Holy Names for a Holy purpose. [Like it says in Perkey Avos 1:13] דאשתמש בתגא חלף – one who exploits the crown [of Torah for his own ends] shall persish. The Rama wrote to this affect “One who exploits the crown shall perish – some say this refers to one who utilized the Holy Names”. The books of the Kabbalists also explain that it is a big sin to use His Name, therefore one who abstains will be blessed.
R’ Chaim Vital writes (Sharey Kedusha Chelek 3 Shaar 6) regarding one who utilizes Kabbalah Maasis nowadays:
“We have a tradition that either he or his descendants will become poor, or sick or non-religious”.