Nuance Recommendations:
The answers to the questions:
1) Can we delete these to free up space
As these are cache files, if Nuance services are stopped and not running, you can delete any/all of them.
If Nuance services are running, you should take care not to delete anything dated more recent than the latest startup time, or anything contained in a sub-folder corresponding to any actively running Nuance process PID.
2) what should be managing this to prevent the problem in the future?
Actually this is an old bug which has had several fixes over the years. In NSS 6.2.7 one such bug was fixed, but you are on NSS 6.2.8 - however, there was one more cache cleanup bug fixed in NSS 6.2.10 please review the release notes.
So, the short term solution is to manually clean up the older cache content when the speech services are down for maintenance, or if done during live operations, then carefully only the cache files that are older than the latest speech services start time.
The long term solution is to upgrade to NSS 6.2.10+
