Please make only the members from JIRA System Administration global permission be able to re-index JIRA. In JIRA 5.2.11 a user with jira-administrators global permission is able to re-index as well.
This had been tracked down on the issue
JRA-23816, which was set as duplicated by the issue JSP-20533. The issue JRA-20533, though, refers to the re-index banner after modifying custom fields, not to the re-index process itself...