If by "lock" you mean write protect them to prevent other users from manually changing them in the Edit Sheet Format mode, I don't believe you can.
If they are engineers, they should be able to understand and comply with a simple "Don't manually change these links" instruction.
If they can't, a 2x4 often works wonders.
As a last resort you could remove the Edit Sheet Format option from the RMB drop-down;
Customize > Menus > Edit > Sheet Format > Remove
I don't have many "thou shalt nots" around here, but that's one of them unless there's some extenuating circumstance. But the best way is to show them that by making the changes in the File/Properties area, they have one-stop shopping. It's faster and easier for them - stress that point. It also saves the shop call when something is forgotten in on eof the blocks as the user is more likely to change all the required properties if they are right there in front of them.
What I do is set the default values for the custom properties in my template to:
"Please enter Number"
"Please enter Description"
"Please enter Class"
"Please enter Revision"
If the user fails to enter the required data, the default value shows up in the drawing title block, so then they'll appear to have been careless, or worse: insubordinate...
Enjoy this day,
One way to keep the 'other engineers' from using the Custom Properties feature (in SW2009+) is to clear the Custom Property Files folder (System Options\File Locations) on just THEIR computers.
If you want to restrict JUST the Drawings, then create an second folder with just the part & assembly property templates (.prtprp .asmprp NO .drwprp) and point to that folder.