The short of it is, this command (inside of weechat) will cause the aligning of nicks to stop. This is helpful for when you're in a channel where there's the one user with the incredibly long name.
/set weechat.look.prefix_align none
Note that the bar (or whatever your prefix_suffix is in your weechat configuration file) will not be displayed.
If you decide that you do not want weechat to remain this way, the margin can be set back with
/set weechat.look.prefix_align "default"