Every time I launch a particular application I have to go into my Users & Groups prefs and remove it from my startup items. How do I prevent applications from automatically becoming startup?
Without specifying the exact application, this answer is only going to be generic.
However, if removing an application from your login items doesn't prevent it from reappearing as a login item (or at startup), usually this means that the specific application's preferences are set to startup at launch, or login, etc. So you would need to go into the applications preferences and search for something along those lines and then disable it.
If this doesn't help, please clarify your question by including the specific application (and version) and also what version of macOS you're running.