It is never nice to see that someone has chosen to vandalize your property. I see main options here.
- Leave it alone. (Already ruled out, since you wish to deal with it.)
- Remove the paint.
- Paint over it.
If you do choose to remove the paint, a paint stripper is a good start. I like the citrus based ones, as they seem less toxic than the old solvent based strippers. Brush the stripper on, let it stand for the indicated time, then wash off, probably with a good stiff brush as an aid. An issue here is you will essentially end up cleaning the surface where you just worked, so this action will tend to leave a negative of painted region. The wall will be lighter in those spots, because it is cleaner. So you will then want to wash down the entire wall. A pressure washer will help, and you can get mild cleaners to help in this part.
The final choice on my list is to paint over it. A good exterior paint will cover the graffiti well enough. (Of course, a downside to paint is that once you paint that wall once, you will be forced to paint it again in the future when the paint gets old.)
An alternative to a blank white wall here is to do it as a mural if you have an artistic bent. (My wife surprised me one day with her artistic talent, when I came home to find she had painted the block wall on our house with a field of cat tail reeds under a bright yellow sun.) How about a nice field of sunflowers against a blue sky? Or, perhaps you might paint the wall to look like a brick wall, but add several fake stained glass windows on this facade?
If you do decide to paint the wall, you might still want to wash it down first to remove any crud that will prevent good paint adhesion. Give it a couple of days to dry before applying paint, especially if it is oil based paint.