Learning Martial Arts is not a disfellowshipping offense, but strongly discouraged in the Watchtower and disqualify a person from being able to hold positions of privilege.
Pay Attention to Yourselves and All the Flock p.141
"It is not in harmony with the spirit of 1 Corinthians 13 : 4-7 or Isaiah 2 :4 for a Christian to learn karate, judo, or other martial arts .A brother would be disqualified as an elder, a ministerial servant, or a pioneer if he took up the study of such martial arts and practiced them . A person publicly advocating such could be dealt with in line with 2 Thessalonians 3 :13-15 . As in the case of a person who takes up professional boxing and continues in such a course, he would be expelled from the congregation . (w81 7/1 pp. 30-1)"
Awake 1995 Sep 22 pp.12-14 Should I Learn Self-Defense?
"There is a greater danger, however, in learning the martial arts. The Economist magazine recently reported: "Most, if not all, of the martial arts are inextricably linked to the three main East Asian religions, Buddhism, Taoism and Confucianism." Another source adds: "Everything done in karate-every movement, every feeling-can be traced to some principle of Zen." Zen is a sect of Buddhism that emphasizes religious meditation. These religious roots pose a serious problem for Christians in view of the Bible's words at 2 Corinthians 6:17: "‘Therefore get out from among [false worshipers], and separate yourselves,' says Jehovah, ‘and quit touching the unclean thing.'""
Awake 1984 Dec 22 pp.12-16
"In time I realized that what I was learning from God's Word did not agree with my love for karate. For one thing, Paul's words that "bodily training is beneficial for a little" conflicted with all the time I was devoting to sports.-1 Timothy 4:8.
Further, as the Witnesses emphasized to me, karate has as its objective hurting an opponent, which can kill or seriously injure a person, even unintentionally. Although this was true, still it was hard for me to accept. How could I forgo all that I had pursued so zealously for 12 years?"
Watchtower 1983 Jul 15 p.25 "Seek Peace and Pursue It"
"However, for private self-defense, might not the Christian take training in the martial arts, such as the Chinese Kungfu? Let it be noted that this most lethal of the Oriental arts was developed more than 1,400 years ago by Zen Buddhist monks of the Shaolin Monastery on the slopes of Songshan, one of China’s sacred mountains. From this religious source came also the martial arts of Japan—Bushido, meaning, literally, "The Way of the Warrior." Many experts in the arts of judo, kendo and karate still draw inspiration from religious meditation. Karate has as its objective incapacitating the victim, which could result in serious injury or death. Surely, those who trust in Jehovah would not turn to the martial arts for defense!—Proverbs 3:31.
If a sizable number of Witnesses in a congregation should be disturbed in that someone is hunting animals merely for sport, and not for obtaining food, he might not be in position to receive special service privileges, because of his offensive reputation.—1 Timothy 3:2.
It is similar if one of Jehovah’s Witnesses insists on carrying or having firearms for protection against humans, or on learning the martial arts. The spiritual elders should take immediate steps to counsel and help him to remedy the situation. (Micah 4:3) Anyone who thus continues to carry personal arms or otherwise equips himself to become "a smiter" would cease to qualify for special privileges in the congregation.—1 Timothy 3:2, 3."
Watchtower 1981 Jul 1 pp.30-1
"It is not in harmony with the spirit of 1 Corinthians 13 : 4-7 or Isaiah 2 :4 for a Christian to learn karate, judo, or other martial arts. A brother would be disqualified as an elder, a ministerial servant, or a pioneer if he took up the study of such martial arts and practiced them."
Awake 1979 Oct 22 p.11
"The so-called martial arts, like judo and karate, are another increasingly popular method of defending oneself. These martial arts train a person to use his hands and body to either injure or kill an opponent, so, in reality, they, too, are weapons. Their pagan background and military usage also raise questions as to their propriety for Christians. Can you imagine the apostle John defending himself with karate chops or Jesus Christ fighting off an opponent with kung fu tactics? (See the December 8, 1975, issue of Awake!, page 28, for a more detailed discussion of this subject.)"