Confine the FAQ, that deals with a similar problem - though I do not know what exactly do you mean by "disable connections" without dropping them?

How to get full control over a SQL Anywhere server?

Basically, you will need to call

call sa_server_option('ConnsDisabled', 'YES')

or

call sa_server_option('ConnsDisabledForDB', 'YES')

to disable new connections temporarily to the server (or database), and will have to drop existing connections (I don't think there's a way to simply "disable" them - however, you might set their priority to "Background".)

see also the question How to limit the remote access to the database server?

As you state that the requirement to have just an exclusive connection is during unload/reload, I do assume that you are about to rebuild or migrate a big database, and you cannot afford to have the database unavailable for that timespan. (I do assume that as it is much easier to simply take down the database, do the rebuild/migration, and make the new database available again.)

If my assumptions are true, a different approach which doesn't need to explicitly disconnecting users, is as following:

• Take a full image backup of the database (with renaming the log) while the database is running as usual.
• Make the rebuild/migration with the backup (or a copy of that) - during that timespan, users can still work, and their operations will get logged in the new transaction log.
• Stop the old database, take the current transaction log and use DBTRAN to translate the log, so you get a list of all those transaction that have been done in the meantime, and apply that to the new database. - That's the (hopefully very short) time when there's no database available for your users.
• Make the new database available for your users.