I can't help you with deleting the existing data, but this will most likely help going forward:
On each Data Collector in the directory $IMROOT/apache-karaf-2.3.0/etc, create a new file named "com.ca.im.dm.snmp.collector.SnmpCollector.cfg", and insert this single line:
showGapsOnCounterRollover=true
The parameter is picked up automatically, so no restart is required.
By default the Data Collector tries to be smart about calculating counter roll-overs. This is a great idea and good intentions on CA's part -- however, SNMP agents have the potential to be buggy and we came across many vendors/platforms that like to roll-over at random times. The Data Collectors happily calculate the amount of data that would've been needed to account for a legitmate roll-over, and thus you're left with 15,000,000,000%.
We've been in really good shape since applying this parameter, as we would rather see a gap in data than bogus data.