Place a stone on an empty Go board
C, 212 195 193 181 171 132 103 98 bytes
Saved 1 byte thanks to @FryAmTheEggman, 5 bytes thanks to @orlp
Call f() with the position to play (must be capitalized), and it prints out the resulting board.
i;f(char*p){for(i=380;i--;)putchar(i%20?i^20*atoi(p+1)+64-*p+(*p&gt;72)?i%6^4|i/20%6^3?46:42:79:10);}

Try it on ideone.