It might mean something like 'don't let them stop you being miserable' or it could be a play on words from 'rain of ruin'. There are a few examples on the net about not letting the sun ruin your rain, which has, to me, the idea of wanting to be miserable. It's not a phrase in common usage, though, so there could be other interpretations. Can you remember anything about the context?
Now, I get it!
I saw that phrase in a T-shirt my friend gave me and it is written exactly this: Don't let them ruin your rain. There is also an image of a gothic girl with a grumpy expression and she is holding an umbrella. It seems to make sense, doesn't it?
I was just curious to know. It is not a good idea to wear a T-shirt in which there is something written and you don't know what it is...