If you have an autoloader or a library, the best thing to do is to partition your device.
What this does is create logical partitions that you can direct your specific jobs too. This then means your jobs only use those slots...
Go into Devices in BEWS, right-click your device, and then choose how you'd like your partitions (1 slot, 2 slots etc). Once done, it will inform you it's creating the partitions and you can click OK.
Then go into each job, and point that job to a specific slot. They will not move onto any other slot as a result.
Let me know if you need further help!