How do I unselect items in GIMP?

I'm currently using GIMP to add text captions and lines into a screenshot. The problem is that once I finished one caption I couldn't add any more lines (using the Paint Brush tool). If I close the GIMP window and reopen it, I can add more lines with no problem, but as soon as I start adding captions these dotted boxes appear and I can no longer add any more lines. How do I overcome this problem?