Looks like you're in luck Yiannis. The filters I created for AdBlock & AdblockPlus seem to work with uBlock Origin.
If you click the uBlock Origin icon on your browsers toolbar, you should see three cogs to open the dashboard, click that.
On the dashboard page, click the My Filters tab at the top.
In the main window of the My Filters tab, paste in the filters that are in the post of mine that I linked to earlier.
Where it has scotthope|anothername in the filter, remove that and put in the names of the users you want to block. Each username must be separated by a pipe character '|'. I don't know where that would be on a Greek keyboard, but on mine it is above the backslash, next to the left shift key. Capitalization when entering usernames into the filter doesn't matter.
Once you've got the filters in place, click the apply changes button, and you're good to go.