How to turn off ALT + numeric keypad ASCII symbol insertion?

You can use Autohotkey to remap, e.g., NumpadLeft to Left, which should remove any of the Alt + Numpad side-effects.

; For Alt + key (with NumLock OFF)
NumpadIns::Ins
NumpadEnd::End
NumpadDown::Down
NumpadPgDn::PgDn
NumpadLeft::Left
NumpadClear::
NumpadRight::Right
NumpadHome::Home
NumpadUp::Up
NumpadPgUp::PgUp
NumpadDel::Del