Welcome in Processing land. Please, take the time to read the section descriptions. Processing Implementations is for alternative languages, like JavaScript or Ruby. As you don't mention a language, I assume it is for plain Processing.

Also I advise to browse through the examples coming with Processing, they often answer such simple request (or not, I haven't checked for this one, I admit...).
Lastly, when asking this kind of question, it is better to show what code you have so far, so that we don't discuss in a theoretical ground, but on a more concrete one.
Basically, to stop a moving object, you just stop updating its coordinates. Using, for example, an if condition to see if the obstacle is reached.

Thanks, would do some search now