As you mentioned it is your competitor, you have no way to contact him or ask him to delete. These ways can help you when find negative reviews.
A quick look at all the negative reviews
Negative reviews are not always bad, if they are not malicious, they are the voices of users. If you take a quick look at all the negative reviews, you can know they are saying and what do they hope to see in your app. You can also find some good ideas and useful suggestions, which can help you to rethink and improve your app to be a better one.
Avoid being defensive
Even if you find some malicious or irrelevant reviews, don’t be defensive, instead, calm down, think about what they said, is that true? Do the problems or bugs really exist? If they are all true then you should be glad that they found the bugs for your app.
Make an apology
Basically, no one wants to get in the endless controversy with a unsatisfied customer. No matter what they said, just make an apology politely and reply them like “Sorry for the unpleasant that caused to you, thank you let us have the opportunity to fix this for you , please let us know if there is anything else we can do to make your experience with us a pleasant one!”
This could stop them to post negative reviews again and may change their mind to change reviews.
Release an update
Releasing an new updated version with all bugs fixed, add an pop-up window to inform the users like “the bug fixed and stability improved”, ask them to update the new version, and add a popup to window in the new version to ask them to re-rate and re-write reviews for your app. This action may help to reduce the negative reviews.
Get better reviews
If you did everything you could, but you still cannot change the negative reviews for your app, then focus on the quality of the app and make it a great app, which will drive more organic 5-star rating and good reviews in the future.