I had this happen to me a while back, about a year or so ago maybe a bit longer and the issue was search bots hitting our site like fire ants .... the main ones were:
AhrefsBot
Baiduspider
Ezooms
MJ12bot
YandexBot
I used
Ban Spiders by User Agent to help with this issue ... now these bots and others don't hit the search feature which is something others around here will say doesn't happen with bots ...
If it worked for me I am sure it will work for you... there is also other things you can do to limit and reduce the amount of hits on your search feature.... you also want to check your mods to see if they are updated .. some modes can attract unwanted activity and actions on your forums ....
And about upgrading .... if you are careful with your site, forums and stay on top of security and suspicious behaviour you don't need to upgrade at all ... I don't mean to insult any of the previous individuals who say you should upgrade but it is not necessary to do so at all ....