did you use trjconv command? with a -nopbc flag, it should work.

http://manual.gromacs.org/documentation/2018/onlinehelp/gmx-trjconv.html

Removing the box information would be:

pbc set {0 0 0} -all

But I guess you just want to deactivate the display of the box, so this is:

pbc box -off

Maybe pbc box -color black

Save your visualization state as vmd file. Then, open the file using text editor, delete the box section.

For example:

graphics top line {0.000000 0.000000 0.000000} {49.169998 0.000000 0.000000} style solid width 3

Finally, you can start vmd, load the visualization state.

Good luck.