what should i do about a male dog getting into our back garden?

[Amanda]

we own two female lab cross sisters who happily play together in our fenced in back garden however every time they are out playing or doing there Business we find a male dog in there the male dog which we think is a yorky cross he is very small by the way.we have tried telling the owner about it and by the looks of things she doesn't care about her animals we have threatened to ring rspca about the dog who is so skinny so we have had to start feeding him poor thing problem is we are scared to ring the rspca as the owners son is in a local dangerous gang please help we are stuck with idea's