Don't worry, you can't really do it by accident.
You use the mixing console to assign an effect to a part. When you first select an effect for a part, the system will select one of the available DSP e.g. DSP5
If you just replace the selected effect by touching it directly on the display, then it will replace the assigned DSP with the new one. If you want to stack the effects, you have to explicitly open the window that assigns DSPs to parts, and assign an additional DSP to that same part.