Clearing out [model] db
If you took a backup prior to adding objects to it you can just restore the backup. If you do not have a backup to restore from, you could take a backup of one that is empty and use that as your restore point. 

Alternatively - you could delete all of the objects that you created within it. Check out some of the answers to this question for ideas around scripting this.

Failing all of the above, you might have to rebuild your system databases.