Pretty sure the sperm dies long before that happens, but if every single girl wakes up with Sensei's baby batter dripping down their thigh, a lot of them may be too disgusted to ever talk to him again, which plays into Ami's wants.
Ami seems to be freezing it in sealed mason jars. It probably wouldn't work well in real life, but it's not like Sensei is a normal human being, or Kumon-mi a normal place.
Doing something like that would probably just allow the girls to blackmail him or ruin his life, though, which I don't think Ami would risk. She seems to already be wary of them, because of their age:
Any of these girls could probably ruin Sensei's life already (at least if the resets weren't a thing).
If Ami really wanted most of these girls to stay away, all she'd probably need to do is show them what's on his computer.