Just a couple more suggestions - both potentially free.
If you have Pages... you can create a box, size it and angle it to the angle of the plate and then fill with the same colour of the plate. Less obvious than thick black line. You can also adjust the opacity to "blur" the plate instead of just obliterating. Save as a PDF, which you can open in Preview and save as a JPG, TIF, PNG, etc. This works for the occasional edit, as it's a bit cumbersome... but if you have Pages already it's free.
If you already have Lightroom 4, use the localized editing brush and "brush" the plate. Then play with the clarity and exposure sliders to get the look you want. If you only need to do a limited number, then you can use the trial version of Lr for 30 days for free.