also u can doit by command: !set (vx or vy) (1-infinity)
I'd also reccomend looking at the wiki if you want more details: https://powdertoy.co.uk/Wiki/W/Main_Page.html
element PROP - select vx and vy properties, 0 is not moving at all. If you set vx to 0, it will be not able to move right/left. Increasing it will make it move faster, also set life to 0 to make particles immortal