This should be no bigger problem:
- Export the members you want to keep (checking them in phpmyadmin in the table exp_members), than export them. you should get something like
INSERT INTO `exp_members` VALUES(4, 5, 0, 0, 0, 'test', 'asdfasdf', '92429d82a41e930486c6de5ebda9602d55c39986', '893a1f8c0215bbea389044bf54e8bb53f9bd1780', 'R0kwA8nOgk', '[email protected]', '', '', '', '', 0, 0, 0, '', '', '', '', '', '', '', 0, 0, '', 0, 0, '', 0, 0, '', 0, 'y', 0, 0, '91.19.246.115', 1201387312, 1201387381, 1201387381, 0, 0, 0, 0, 0, 0, 0, 0, 'n', 'y', 'y', 'y', 'y', 'y', 'y', 'y', '', '', 'n', 'n', 'us', '', '', '', '', '28', '', '18', '', '', 0);
for each row you want to keep.
- In phpmyadmin just empty the table exp_members
- import the exported members (in the left column of phpmyadmin click on “exp_members”, then at the top the tab “sql”, paste the sql-code)
for sure you should not forget to keep a complete backup of the database in case something goes wrong.
good luck!