From what I know, it's not possible to cancel an unconfirmed transaction. The closest thing would be a dojble-spend attack. Seeing as you sent it without a fee, it will most likely be returned to your wallet within a few hours/days.
No! OP is using bitcoin core. Core does not "return" transactions, it constantly rebroadcasts them if they have been created with the wallet.
Just be patient and it'll come. On the other hand, it could confirm after a while but your only option right now is to wait and be patient. Always remember the fee, I know I've sent a few transactions without a fee and it took forever to get the Bitcoin transaction confirmed. Could you possibly post a txid so we could try to rebroadcast it or just check on the status?
A TX ID would be nice, yes. It will most likely show a low fee though.
@huberius you have two options
#1 keep core open and hope the TX will get picked up. This will happen eventually, but it might take days or weeks. Its difficult to say without knowing the details of the TX. It might be possible to get a confirmation even without fee, depending on the priority.
#2 remove the unconfirmed transaction from your wallet (more below) and wait another 24 hours so the rest of the network had a chance to forget about it. Create it again, this time with a higher fee.
To remove the TX from your wallet (assuming you dont run a pruned node!)
#1 close core
#2 make sure its closed
#3 start it with
-zapwallettxes (make sure the spelling is correct), for windows you do this by:
#3.1 opening run (win + r)
#3.2 enter:
c:\Program Files\Bitcoin\bitcoin-qt.exe -zapwallettxes (modify path if needed)
#3.3 confirm with ok
#4 core will forget about all transactions and rescan the local blockchain. This will re add all confirmed transactions, but not the unconfirmed one.
