There is nothing more irritating to a visitor than a site that appears to be trying to control them, either by forcing them down particular paths, of by disabling features of their browser interface.
Here are a couple of examples of the sort of thing I am talking about:
Opening new windows, so that the original site is still hovering in the background; if the visitor wants to come back to your site they can click the back button and if they want to open a new window they can do it themselves.
Using scripts to disable the back button, disable right clicking, or subvert some other aspect of the browser interface.
These techniques can decrease accessibility as they go against users expectations about how the web works. I expect to be able to click the back button in my browser to move back to the last page viewed; and I don’t expect my back button to suddenly stop working.
Trying to control the visitors to your site is a bad thing; not only can it make your website less accessible it can also make you seem less credible as an organisation; that’s what happens when things on your site don’t work as expected.