Turn on the developer console (go to Settings - Game Settings)
Open the console [~]
Paste these commands into the console:

bind "shift" "+speed;r_cleardecals";
bind "w" "+forward;r_cleardecals";
bind "a" "+moveleft;r_cleardecals";
bind "d" "+moveright;r_cleardecals";
bind "s" "+back;r_cleardecals";
bind "MOUSE1" "+attack;r_cleardecals";
bind "TAB" "+score;r_cleardecals"