> 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.
Ok, so I've already deleted few files, maybe that's why it started again..
Basically I'd just need to transfer the bitcoin folder which is currently in Application support to an external harddrive ? Then I can delete the one in my desktop without any risk ?