With Bitcoin Core... you'll want to try this:
- Start Bitcoin Core with the command-line option -walletbroadcast=0. On Linux, you might be able to just run bitcoin-qt -walletbroadcast=0, depending on how your current startup script works. On Windows: find the shortcut for Bitcoin Core on your desktop or start menu; right click it and choose "properties"; add -walletbroadcast=0 to the end of "target", so for example "C:\Program Files\Bitcoin\bitcoin-qt.exe" would become "C:\Program Files\Bitcoin\bitcoin-qt.exe" -walletbroadcast=0; click "Apply"; use that specific shortcut to start Bitcoin Core.
- On the Transactions tab, right click the stuck transaction and choose "abandon transaction". Warning: Even though the transaction is listed as abandoned, it can still go through. People have in the past lost money by abandoning a transaction, resending a separate replacement transaction, and then having both transactions go through. The following steps are designed to replace the abandoned transaction in a way which will prevent this sort of double payment from happening.
If "abandon transaction" doesn't work... you can always try -zapwallettxes... That will definitely remove it from your wallet and stop the transaction from being rebroadcast.
Note that these commands are likely to result in wallet rescanning which could involve a lot of time.