I have never understood why people use the Hide Lines feature of UltraEdit when next command is Delete All Hidden Lines.
It is much more efficient to run a regular expression replace from top of the file which deletes directly all lines containing a string.
For example with Perl regular expression engine searching for ^.*string.*\r*\n and using an empty replace string finds entire DOS or UNIX terminated lines containing anywhere inside the line string and deletes those lines.
Please note that the last line of the file must have also a line termination if containing string or it will not be deleted.
If the file contains most likely consecutive lines containing string, it would be even faster to use as search string ^(?:.*string.*\r*\n)+ to find not just lines containing string, but a block with 1 or more lines containing string to delete the entire block at once instead of each line separately.