Thanks for the reply. I tried editing /etc/default/console-setup and changed ACTIVE_CONSOLES="/dev/tty1" but I can still switch to tty2,3, and 6 by pressing the corresponding Fn keys while holding alt. Tty4 and tty5 seem to be disabled, so maybe I'm missing something and the other 3 are being activated at login?

I also edited NAutoVTs=1 in my systemd logind.conf, but this also hasn't stopped the additional TTYs from appearing.

[UPDATE]
Had a feeling the TTYs still active were being started somewhere at boot. Checked my /boot/cmdline.txt and changed console=tty3 to console=tty1 and now Alt+F3 is disabled!

Still need to disable Alt+F2 and Alt+F6. Any suggestions are appreciated!