If you sent the tx using a standard (non-raw tx API) method then in all likelihood the tx is valid and will eventually get confirmed (this could take quite a while depending upon the priority it has). For this case "undoing" the tx is not possible (so be patient and maybe next time consider adding a slightly bigger fee).
If the tx is actually not valid (which should usually only possible if you had constructed it yourself manually using the raw tx API) and you are certain it will never get confirmed then you can use pywallet to remove the unconfirmed tx from your wallet (this can be necessary for removing double-spends).
Topic: can an unconfirmed transaction been undone? (Read 860 times)
so, i have a transaction which doesnt seem to confirm due too many small inputs and no fee (electrum client). can it be undone?
Jump to: