following musicamante suggestions I modified my code with:
def keyPressEvent(self, event):
    if self.keyonoff:
         print(keyevent_to_string(self, event))
         print('self.letters : ',self.letters)
         print('self.letters_guessed : ',self.letters_guessed )
         if keyevent_to_string(self, event) in self.letters:
                    print('inininininini'*5)
                    self.printo(keyevent_to_string(self, event))
then switched 
self.keyonoff = True and   self.keyonoff = False
as needed by my code.
I am getting what I was looking for, not sure if there is another way to
do that: 'I understood that KeyPressEvent is the handler for QEvent.KeyPress, so is there a way to disable/delete/remove QEvent.KeyPress class in Pyqt5 ?'
Code below is not mine is just a keyPressEvent reimplamantation usefull to my purpose
from PyQt5.QtCore import Qt
def keyevent_to_string(self, event):
    self.keymap = {}
    for self.key, value in vars(Qt).items():
        if isinstance(value, Qt.Key):
            self.keymap[value] = self.key.partition('_')[2]
    self.modmap = {
        Qt.ControlModifier: self.keymap[Qt.Key_Control],
        Qt.AltModifier: self.keymap[Qt.Key_Alt],
        Qt.ShiftModifier: self.keymap[Qt.Key_Shift],
        Qt.MetaModifier: self.keymap[Qt.Key_Meta],
        Qt.GroupSwitchModifier: self.keymap[Qt.Key_AltGr],
        Qt.KeypadModifier: self.keymap[Qt.Key_NumLock],
        }
    self.sequence = []
    for modifier, text in self.modmap.items():
        if event.modifiers() & modifier:
            self.sequence.append(text)
    self.key = self.keymap.get(event.key(), event.text())
    if self.key not in self.sequence:
        self.sequence.append(self.key)
    return '+'.join(self.sequence)
From Get printable name of any QKeyEvent key value
Edited Edited Edited
More on this hopefully I'll get my right to ask questions back:
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
Created on Fri May 29 10:49:58 2020
@author: Pietro
"""
import sys
from PyQt5 import QtWidgets
from PyQt5.QtWidgets import QApplication, QWidget
from PyQt5.QtCore import Qt, QEvent
class Window(QtWidgets.QMainWindow):    
    def __init__(self):
        super(Window, self).__init__()
        self.button = QtWidgets.QPushButton("activate", self)
        self.button2 = QtWidgets.QPushButton("deactivate", self)
        self.button.move(0,0)
        self.button2.move(100,100)
        self.button2.clicked.connect(self.deactivated)
        self.button.clicked.connect(self.activated)
    def activated(self):
        print('attivato')
        QWidget.setEnabled(self,True)
    def deactivated(self):
        print('deattivato')
        QWidget.setEnabled(self,False)
    def keyevent_to_string(self, event):
        self.keymap = {}
        for self.key, value in vars(Qt).items():
            if isinstance(value, Qt.Key):
                self.keymap[value] = self.key.partition('_')[2]
        self.modmap = {
            Qt.ControlModifier: self.keymap[Qt.Key_Control],
            Qt.AltModifier: self.keymap[Qt.Key_Alt],
            Qt.ShiftModifier: self.keymap[Qt.Key_Shift],
            Qt.MetaModifier: self.keymap[Qt.Key_Meta],
            Qt.GroupSwitchModifier: self.keymap[Qt.Key_AltGr],
            Qt.KeypadModifier: self.keymap[Qt.Key_NumLock],
            }
        self.sequence = []
        for modifier, text in self.modmap.items():
            if event.modifiers() & modifier:
                self.sequence.append(text)
        self.key = self.keymap.get(event.key(), event.text())
        if self.key not in self.sequence:
            self.sequence.append(self.key)
        return '+'.join(self.sequence)
    def keyPressEvent(self, event):
            print(self.keyevent_to_string(event))
if __name__ == '__main__':
    app = QApplication(sys.argv)
    window = Window()
    window.setGeometry(600, 100, 300, 200)
    window.show()
    sys.exit(app.exec_())
I ended up here 
QWidget.setEnabled(False)
apparently disable both keyboard and mouse events. Any idea how to disable just keyboards events ?