How do I stop getting spam emails from dating?

Use the Google Block Sender feature Block or unsubscribe from emails and Block or unblock peoples accounts, and in this case they also go to Spam. If using an Android device you can click on the three horizontal dots in Gmail app, to make similar Block/Unsubscribe settings.

Why am I suddenly getting hundreds of spam emails?

If you start receiving an increased amount of spam, with junk mail filters enabled, then there might be a problem with the mailbox that your spam emails are usually moved to. You should check that the target mailbox or mail folder isnt full or disabled.