If the administrator has enabled custom blocks on the site you're using, you will be able to disable or enable the blocks the administrator has chosen by going to your account settings. To do this:

1. Login
2. Click "my account", then the "edit" tab
3. Scroll down to Block configuration. If you do not see this option, then the administrator has not designated any blocks as custom.
4. Check the boxes for the blocks you want, uncheck the blocks you don't want
5. Click "Save account"

Note: this will only work for blocks that are designated as custom blocks. There may be sidebar blocks that the administrator wants to appear for all users, and you will not be able to disable these, so they will not listed in your account settings.