How to hide admin account and only show main user at logon screen in Windows?

Yes, it seems you can hide the account following guidelines explained at Microsoft Technet forums. However you can't login then unless you set the registry key back.

1. Go to regedit
2. Go to HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\Winlogon
3. On the left, right click on Winlogon, click New and click Key.
4. Type SpecialAccounts and Enter.
5. On the left, right click on SpecialAccounts, click New and click Key.
6. Type UserList and Enter.
7. In the right panel of UserList, right click on a empty area and click New then click DWORD (32bit) Value.
8. Here, type in the name of the user you want to hide. You may have to experiment with that, I don't know how spaces or changed user names affect this.
9. Right click on the user account name and click Modify.
10. To hide the user account type 0 and click OK.

If you want to show it again you have to enter 1 instead of 0 at the last step.

Alternatively, you can see the answers from this post: "Hide account from login screen but can be used in UAC"