I second this notion if there is a problem with the plot that is making you not enjoy a story. Sadly, if your problem lies in other areas, such as how well your partner can type proper English, then there might be nothing that you can do about that. Noone here is perfect and, even with spell check, errors still manage to get by us all. There is a signification difference, however, between the occasional misspelled word or run on sentence and being put into a position where trying to read your partner's work is like trying to decipher ancient hieroglyphics.
In these instances, it is best to just be straight with your partner and tell them that their posts are hard to read, tell them what the issues are, and give them a chance to improve. If, after a couple more posts, you still cannot enjoy the RP because you still cannot read it very well then it might be time to refer back to Rudolph's advice and just be straightforward, telling them that you are not interested in continuing.