Don't quote me on this but I'm pretty sure the mempool on Core doesn't persist the process, so simply closing Core should do it. You should still ask Core devs directly on their slack/IRC channel.
Yes, that is all you need to do. Restart the node and the mempool will be cleared.
So, all i have to do is help->clear all unconfirmed transactions on armory, or what do you mean by restarting the node?
You do that to remove the transaction from Armory.
Or do i have to do something else with bitcoin core / bitcoind ? Do i have to close the process bitoind.exe?
You restart Bitcoin Core to clear the mempool and that will stop the transaction if it never hit the network. AFAIK, Bitcoin Core does not rebroadcast transactions in its mempool.