There are so many pre-programmed in and is hard to lock all but the ones that are an interest to you.
I didn't think the 246 came with pre-programmed frequencies

If you bought a used radio, or had someone else program it, a better solution would be to delete the systems you don not want. That way, you will have all the Quick Keys available for the systems you want.This comes from
Easier to Read BC246T Scanner Manual, which you may find quite helpful :
To Delete a system, press Menu. At 'Program System' press E/Yes.
At 'Select System' scroll to the system you want to delete and press E/Yes.
Scroll to 'Delete System' and press E/Yes.
At 'Confirm Delete?' press E/Yes.
Press Scan to exit.