Yes, you can always try! Picasa has a tool called the “Retouch” tool which should do the trick.
From Google:
Use the Retouch tool to remove unsightly blemishes and improve photo quality. It can also help you to restore old photos with marks, water stains, and scratches to excellent condition. Use the following steps:
Click the Retouch button.
Click once to highlight the area you’d like to retouch. A few tips:
Adjust the size of your brush using the slider (or use the + and – keys on your keyboard).
Use Ctrl-drag to pan across the image, especially useful when viewing your image while zoomed-in.
Use your mouse wheel to zoom in on an area you indicate with your cursor.
Move the mouse around the image to find and preview a replacement area.
Click on the replacement area to finalize.For natural looking results, use the smallest brush size, and pick a replacement area close to the photo blemish for a better tonal match.
I also found a video tutorial that shows how the ‘Retouch’ tool works. https://www.youtube.com/watch?v=rShTlLCZ4MY
My opinion is that the lens flare is actually rather artistic, and in some cases I like to leave it in if it looks good. Not all lens flare is equal though, so of course, it’s up to you whether or not to remove it.