While there is a subtle difference between stop, prevent, and hinder (which I think is the usual translation for 阻む), the subtle difference may actually be a difference without a distinction. Did the robbers just hinder your exit, or did they physically stop you from leaving? If they stopped you from leaving, 止める works fine. (You could conceivably construe a sentence with 阻止, but I think 止める, would be a better choice).
Same for the rain sentence. We wouldn't normally say the rain obstructed us from going to the park. We might conceivably say the rain hindered us from going to the park, but it does sound a bit like you made it to the park despite the rain. I think normally one would say, "The rain stopped us from going" or possibly, "We couldn't go because of the rain". It would be odd to say in Japanese, "雨が公園へ行くのを阻止した". Maybe understandable, but not natural, I think. Normally one would just say, "雨のせいで(悪天候で)公園へ行けなくなった" or some variation on that.