Disallow Changing Boot Option and Bash-Like Command Line in Grub

Is there any way to block them from changing or having a command line in grub?

You can set and then encrypt a grub password:

Use grub password command in grub.conf

/boot/grub/grub.conf contains information about the entries that are displayed in the GRUB menu during system startup. On some systems, /etc/grub.conf is a symbolic link to /boot/grub/grub.conf

Add the following password line to the grub.conf file.

$ cat /etc/grub.conf
default=0
timeout=15
password GrbPwd4SysAd$
..

Once the password command is added to the grub.conf, the following message will be displayed right under the GRUB menu during the system startup.

Use the up-arrow and down-arrow keys to select which entry is highlighted. 
Press enter to boot the selected OS or
'p' to enter a password to unlock the next set of features.

You should then encrypt the grub password:

Encrypt the grub password using grub-crypt

grub-crypt will get the clear text password from the user, and display the encrypted password as shown below.

# grub-crypt
Password: GrbPwd4SysAd$
Retype password: GrbPwd4SysAd$
^9^32kwzzX./3WISQ0C

Modify the grub.conf file, add the password entry with the –encrypted argument as shown below. Just copy the output of the grub-crypt command, and paste it after the –encrypted argument in the password entry.

$ cat /etc/grub.conf
default=0
timeout=15
password --encrypted ^9^32kwzzX./3WISQ0C

..

By default, the grub-crypt command encrypts the password using SHA-512 algorithm. You can also encrypt the password either using SHA-256 or MD5 algorithms as shown below.

# grub-crypt --sha-256
# grub-crypt --md5

You can also use md5crypt to encrypt the password. In that case, you should use password –md5 encrypted-password in your grub.conf file.

Source: How to Password Protect Grub Boot Loader in Linux

What if I'm using grub2?

Generate GRUB Bootloader Password

Create a password for GRUB, be a root user, and open the command prompt, type the below command.

# grub2-setpassword 

When prompted type grub password twice and press enter.

This will generate a hashed GRUB bootloader password in the file /boot/grub2/user.cfg file and can be viewed using the cat command as shown.

# cat /boot/grub2/user.cfg

After creating the GRUB password, you need to re-create the new GRUB configuration file by running the following command.

# grub2-mkconfig -o /boot/grub2/grub.cfg

The above command will set the grub password in the configuration file. Now, reboot the system and check if the new GRUB password is set properly.

# reboot

Testing GRUB Password Protection

After your system restart, you will get the following GRUB screen, where you will get 5 seconds to break the normal boot process. So quickly press e to break the boot process.

Once you press e it will prompt you to enter the GRUB password as shown.

After entering the right username and password, you can edit GRUB parameters as shown.

Source: How to Set GRUB2 Password in RHEL, CentOS and Fedora Linux