Including the URL when asking questions about configuration files makes it easier to give you a good answer.
You can use the greater_than function or increase the center dead-zone with a square dead-zone shape for WASD.
Examples:
| kb_w | greater_than 20 | up |
| kb_a | greater_than 20 | left |
| kb_s | greater_than 20 | down |
| kb_d | greater_than 20 | right |
| kb_w | normal | up |
| kb_a | normal | left |
| kb_s | normal | down |
| kb_d | normal | right |
| joystick_dead_zone_shape | normal | 0 |
| joystick_deflection_minimum | normal | 10 |
These both assume the main preference setting is for
circular dead-zone (joystick_dead_zone_shape=1) and you need to "square" it up for controlling discrete keys with the joystick. Adjust the number after the greater_than or the joystick_deflection_minimum to a comfortable value.
If you try to control WASD directly from the up/down/left/right joystick with a circular dead-zone then two keys will usually activate and the character will always move on the diagonal.