I did as laie_techie done. I've a group "Bots" (ID in the mybb_groups : 10) with restricted rights.
Here's the content of my mybb_spiders table:
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('1','Google','0','','10','Googlebot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('2','Lycos','0','','10','lycos','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('3','Ask.com','0','','10','Teoma','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('4','What You Seek','0','','10','whatuseek','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('5','Internet Archive','0','','10','archive_crawler','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('6','Alexa Internet','0','','10','ia_archiver','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('7','Bing','0','','10','bingbot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('8','Yahoo!','0','','10','Slurp','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('9','Baidu','0','','10','Baiduspider','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('10','Twitter','0','','10','twitterbot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('11','QuerySeeker','0','','10','QuerySeekerSpider','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('12','Crowsnest','0','','10','Crowsnest','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('13','Paper.li','0','','10','PaperLiBot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('14','Showyou','0','','10','ShowyouBot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('15','Flip','0','','10','FlipboardProxy','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('16','Yandex','0','','10','YandexBot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('17','Majestic 12','0','','10','MJ12bot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('18','Linkdex','0','','10','inkdexbot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('19','DataSift','0','','10','TweetmemeBot','0');
INSERT INTO mybb_spiders (`sid`,`name`,`theme`,`language`,`usergroup`,`useragent`,`lastvisit`) VALUES ('20','Ahrefs','0','','10','AhrefsBot','0');
Feel free to complete this list