"Spraying the walls" occurs when a man and a woman are having a prolonged and heavy make out session when at some point the woman decides to end it; leaving the man highly aroused but unsatisfied (see Blue Balls
). Having no other choice, the man has to find a private place to relieve his built up tension which by this time is so intense that when he comes the ejaculate exits his body with such force that he "sprays the walls".
Jane had given Jim such a bad case of blue balls
that when he got home to finally rub one out
he pretty much had to "spray the walls" with his jizz