No this is hard coded into the system.

When XOOPS interpets bb code links it adds target="_blank". So it reads:

[URL=yourlink]LinkText[/URL]

and changes it to:

LinkText in html

This forces a new window to open.

It is possible to edit the file /html/class/module.textsanitizer.php. There is a small function in there called &xoopsCodeDecode that handles this. You can just remove everything that referes to target="blank". (Note: always make a backup before editing code.)

The best way to get around this is to make any custom blocks with links that you want to open in _self or _top in html. Then save the blocak as html.