I know that the Qasm Simulator execute quantum circuits with presence of noise, even if I do not specify the model of noise in options for that backend.
But how can I execute the circuits without noise at all?
This is incorrect. The QASM simulator by default has no noise. The fluctuations in its results are a result of finite sampling of the output statevector. Thus, the QASM simulator is equivalent to running a quantum circuit on an ideal quantum computer. To add noise you can follow the example here: https://www.qiskit.org/aer