Try running yes | someprogram. That's what yes is for, isn't it?

If you're so unlucky to be stuck on a Windows machine with no access to unix tools, you can try echo | someprogram instead. That has the disadvantage of outputting only a single line, so if your program is waiting for more, it may still get stuck: in that case redirect an input file with several newlines to it, such as <\path\to\yes.txt someprogram where you previously create the file yes.txt which contains lots of empty lines.