MD (software) raid checks are performed every Sunday at 1am on Centos6 systems, to stop a current raid check you can run the following command (updating the raid device number to that of your device).
echo "idle" > /sys/block/md0/md/sync_action
When you now view the status of the software RAID device you should see the sync has stopped.
It may have moved onto anothe device if this was delayed due to it sharing a physical device with the previous check - if this is the case you may also want to stop this check.
N.B. stopping these checks may lead to data loss if your raid device needs to be checked or has a fault.