You could copy and paste schema.rb into a migration and back-date it (e.g. change the date) so that no existing databases will run it. After you create this migration you can delete all your old migrations.
I disagree with Andrew that you should never delete migrations. Migrations break unexpectedly all the time based on model classes changing and it is very non-trivial to fix them. Since I’m sure you are using version control, you can always look back in the history if you need them for reference.