Built-in WiFi configured as AP keeps connecting to router

Question

I have carefully followed this guide in order to set up my RPi 3 as an AP. The Pi won't be connected elsewhere so I am not intending for it to provide Internet access. I only want to remotely connect to it, via WiFi through a private ad-hoc network between my laptop and Raspberry Pi.

I can "see" the new SSID on my wireless devices but I'm unable to connect to it. Moreover, if I run iwconfig I notice that the wlan0 is still attempting to connect to my router:

wlan0     IEEE 802.11  ESSID:"MyRouterSSID"  
          Mode:Managed  Frequency:2.417 GHz  Access Point: (...)
          Bit Rate=72.2 Mb/s   Tx-Power=31 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:on
          Link Quality=59/70  Signal level=-51 dBm  
          Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
          Tx excessive retries:0  Invalid misc:0   Missed beacon:0

What else should I do to enable the AP and remove previous settings?

Answer 1

The problem seemed to be caused by previous settings and the WPA supplicant service. Disabling it with:

sudo systemctl mask wpa_supplicant

and commenting the lines in /etc/network/interfaces:

allow-hotplug wlan0
iface wlan0 inet manual
#    wpa-conf /etc/wpa_supplicant/wpa_supplicant.conf

has solved the problem for now.