Is it possible in phpMyAdmin to lock a table in a MySQL database so that no more data can be added to it?
And if so how would I do it?

See this post: StackOverflow and this one: MySQL

Thanks Carefree but having looked into it further I can't lock the table as it needs to be written to when using the software that uses that database. I've made a few changes to the software which hopefully will prevent it from being hacked into again (not Snitz by the way), time will tell

Put a condition on writing to that particular table if it's especially important. Maybe a second password requirement or special ID of some sort.

I've renamed the file extensions for the files that are linked to the admin side so that it's not possible to add additional administrators which would then prevent someone manually altering settings, etc.
However I'm not sure exactly how a hacker gained access to do so in the first place (adserving software)
All I know is that some of the changes would have had to have been done manually, plus two additional admin accounts had been set up to do so.