I am using Krusader 2.4.0-beta3 on Ubuntu 16.04. Currently, the only keyboard shortcut to close a tab in the viewer AFAICT is Esc. This is nice for consistency with file managers like Total Commander where a new window is opened for each file but is unexpected for a tabbed environment and not even indicated in the menu. I think that Ctrl+W should be added as an additional keyboard shortcut, visible in the menu.
Looking at https://github.com/KDE/krusader/commit/92ee349a4817eb0bd069798f07dc1c9f09aa0564, it appears that this has been fixed in 2.5.0 already. If somebody can confirm please resolve this as WORKSFORME.
Actually, I verified myself by compiling 2.5.0 from source. Ctrl+W is now an alternative to Esc and is configurable.