MySQL: can I put the database into a maintenance mode? (i.e. no user connections)
As said by Brad Silva, you can skip connections, but you need to restart the mysql server anyway ( with --skip-networking ), as skip_networking is a read only variable.

When finished, restart mysql without this parameter.

EDIT: When I say it's a read-only variable, I mean you cannot simply set it in the console, like other variables you can switch at runtime. This requires a restart, unless I'm wrong.