delete strings after match to eol using sed command

# You can use awk's field separator (-F) following way:
$ cat file
google.com/funny
unix.stackexchange.com/questions
isuckatunix.com/ireallydo
$ cat file | awk -F '\\.com' '{print $1".com"}'
google.com
unix.stackexchange.com
isuckatunix.com
# To explicitly delete everything that comes after ".com", just tweak your existing sed solution to replace ".com(anything)" with ".com":
sed 's/\.com.*/.com/' file.txt