how can I override alias set in .bash_aliases

Add a \ (backslash) before the command to disable the alias, like this:

\ls

This will invoke the original (un-aliased) ls.

Example:

$ ls #will invoke the alias
total 0
-rw-rw-r--    1 dogbane foo          0 Nov  3 16:04 c
-rw-rw-r--    1 dogbane foo          0 Nov  3 16:04 b
-rw-rw-r--    1 dogbane foo          0 Nov  3 16:04 a

$ \ls #will disable the alias
a  b  c