For zsh
you can use setopt aliases
#!/usr/bin/zsh
alias hoo="echo bar"
unsetopt aliases
hoo # outputs `./test.zsh:5: command not found: hoo`
setopt aliases
hoo # outputs `bar`
see man zshoptions
for detail.
For csh
and tcsh
, sourcing the files (source ${HOME}/.cshrc
, for example) suffices.