I know you previously asked to block the bots and now you just want to slow them down.

The only way to slow them down is not to serve them content if they are going to fast. The problem with this is it will surely have negative consequences to your search engine ratings.

In case you decide to block some of the less important bots, you can block a bot's user agent using Apache's configuration like this:

SetEnvIf User-Agent BadBot GoAway=1
Order allow,deny
Allow from all
Deny from env=GoAway

Just replace BadBot with Yandex or whatever bot you want to block.

When Apache sees the user agent it will pretend like there is no content available.

Keep in mind the User Agent headers are very easy to change so if Yandex changes theirs to something else, this hack won't work.

I know you previously asked to block the bots and now you just want to slow them down.

The only way to force them to slow down is not to serve them content if they are going too fast. The problem with not serving content is it will surely have negative consequences to your search engine rankings.

In case you decide to block some of the less important bots, you can block a bot's user agent using Apache's configuration like this:

SetEnvIf User-Agent BadBot GoAway=1
Order allow,deny
Allow from all
Deny from env=GoAway

Just replace BadBot with Yandex or whatever the User Agent of the bot you want to block is.

When Apache sees the user agent it will pretend like there is no content available.

Keep in mind the User Agent headers are very easy to change so if Yandex changes theirs to something else, this hack won't work.