When you split the database, you have the backend on the server so it is available to all of the copies of the frontend you have distributed (Each user should get a copy of the frontend for them to use). You can then keep a master copy which you can edit and then, send out again. But, you don't have to send it out again if you use something like either my Auto Update Enabling Tools (see my website for the free tool) or using one like Access MVP Tony Toews has created - also free. Then you make changes to the master and the users get the latest version.
As for locking it down, you can
1. Create an MDE or ACCDE (if using the new 2007 format) which protects the code/macros/forms and reports.
2. Use the Disable Bypass Key code which can be found here or on Google.
3. And uncheck the USE Access Special Keys checkbox in the Database Startup options.