If you're using bash shell then check:
cat .bashrc | grep tar
cat .bash_profile | grep tar
If it fetches any entry stating with tar, then simply edit .bashrc/.bash_profile file and put a #
against that line.
Or if using csh/tcsh shell, then do the same with .cshrc or .tcshrc files.
And if still pb persists, then also check .login or any personal startup file.