Either lower your standards a bit and accept them and roll with it, seeing how they go and giving them friendly pointers, hints and help with possibly improving their grammer and spelling and overall writing as you go along, because who knows they might be able to do so, or just be up front and say you don't think their writing standard neccessarily matches what you're looking for in your rp, and wish them the best of luck with their other rps. Personally I would go with the former if it was my rp, because if there's nothing wrong with their actual character and understanding of the rp then I'd be willing to give them a chance and try to help them improve their writing, however if you feel it's that detrimental to your enjoyment of your rp then be upfront and honest about it.