@milo you can remove it a few different ways.
When you first clone the app before starting it, you can remove invoking seeding database by removing dummyData(); shown on line 50 here in server.js. Long as you have at least one post in your collection it won't seed the data base, check the dummyData.js file here
if you already seeded the database as indicated in your question, another way is to remove it is from the mongodb shell via command line. Here are the explanations followed by commands:
mongod
mongo mern-starter
show collections
db.posts.find().pretty()
db.post .remove()
db.post .drop()
Other people use a ui interface such as Robo Mongo, to play without paying just use an older version.