Locked area on slides

Our presentation screen at the church will be shown directly on the wall. Unforunately there is an arch, so we can't use the whole rectangular; the lower left corner will be shown on a wall 6 meters behind.
At the moment, we use Powerpoint or Openoffice with a customized template and set the content of the slides manually for not to write into this "forbidden area".

So what we need for using OpenLP in our church is the possibility to lock areas on the slides for not to be written into that.