Technically, you can't do this EASILY if you only mean for it to happen for a single report and DON'T care about other reports and don't have any other restrictions on who can do what to whom.
BUT...
If you implement workgroup security, you can protect the ability to enter Design mode on reports based on the permissions you place on the individual report objects. With workgroup security, you COULD selectively protect some reports by restricting who could enter design mode for the given reports. Since you need a password to use a particular workgroup account, that is a form of password protection. What this would do is not put a password on specific forms, but put a password on a specific ability and put the ability on specific forms in a two-step process. Then you have to trust the people to whom you give this ability.
Look up "Workgroup Security" in the help files and search this forum for threads with that topic. You'll get just over a bazillion responses on same.