Thanks for posting!
You sure can delete values with a Lua script; Indexing an alias with a given timestamp and setting that datapoints value to nil should delete it.
This would look like:
mydataportAlias[TimeStamp] = nil
Replacing nil with an appropriate value would instead overwrite the value of the targeted datapoint.
More information on the operations available in our Lua engine's Alias table can read here: http://docs.exosite.com/scripting/#the-alias-table
Is this a one-time operation, or will you find yourself doing this regularly? We also support the ability to flush periods(essentially slices) of data through our RPC API. Flush allows you to define starting and ending bounds to delete all datapoints in that window.
Good luck & happy developing,