Right-click in the Build Window and select "Clear Window".
Alternatively, run the command "clear-pbuffer" on the SlickEdit® command line. To bring up the SlickEdit command line, hit ESC in most emulations, or click on the SlickEdit message line.
To have the Build window cleared before each build, go to Project-->Project Properties-->Tools tab-->select the "Build" tool from the list, and check the "Clear build window" checkbox.