You will find the following codes in the Constrain page:

for (int i=0; i  double r = diameter[i]/2;
  double d = y[i]-ymin-r;
  if (vy[i]<0 && d  d = ymax-r-y[i];
  if (vy[i]>0 && d  d = x[i]-xmin-r;
  if (vx[i]<0 && d  d = xmax-r-x[i];
  if (vx[i]>0 && d}

The above codes let particle changes direction when it hit the wall.
The id for main particle is 0, so change the above code 
from [b][u]for i=0[/u][/b]  to [b][u]for i=1[/u][/b]
Then,main particle will not be bounded to the simulation region.
You may add whatever conditions you want into the Constraint page(s).