> could I delete the blocks folders stocked in the data folder in order to create space ?
No, you'll likely corrupt the data directory and you'll have to synchronize again.
> How can i correctly activate the pruning mode ?
If you're not very familiar with the syntax for Bitcoin.conf, you can try activating pruning by going to Settings>Option and checking the checkbox to prune the storage.
> Is there any possibility to get the bitcoins without full syncrhonisation, and send them to another wallet ?
Yes but I highly recommend against this.
> If I do the full synch, how could i stock the blockchain on a external harddrive ?
You can point the data directory at an external harddrive by moving and pointing the data directory at the external drive. Use the config file for that.