You can set the file permissions on existing BDs as read-only for use in other projects. This will prevent the BDs from being inadvertently modified.
If you have generated output products for the BD, you can change the file permissions on all files (using chmod 555 bd -R on Linux).
The BD, and all its output products, will be read-only. Synthesis, simulation, and implementation can be run using these files.
However, if you have not generated output products for the block design (BD), you can still make the BD file read-only (using chmod 555 bd/design_1/design_1.bd in Linux). From this read-only you can still generate the output products needed for the design, but the BD itself cannot be changed. You can generate the output products for read-only BDs, if they have not been previously generated, provided the BD has been validated and saved.
Typically, for read-only BDs, either a user managed wrapper file or a Vivado managed wrapper file is already generated. That wrapper file should be added to the project along with the BD.