Easy way to do that. Would be installing world-guard, and creating a flag on the area You want the spawn to be.
/rg flags {flag You created} block-break/place deny.
Requires World Guard
Do You mean "playerserver" that is the only default plugin that can not be deleted.
However, You can set flight in settings to on, so It should not kick you. Dashboard -> settings -> flight
Im scared, that this plugin is not up-to-date, and won't work on 1.16.5. However I found an alternativte for that.
(https://www.spigotmc.org/resources/lightningstrike.83076/) - take a look at that.