Feature summary (what you would like to be able to do and where):
A new block option is added to the block form to prevent the user from sending purge requests.

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
There is a robot on my wiki which does nothing but send purge requests after registration. The only way to deal with this now is to block the user completely, but in some cases it is extreme.

Benefits (why should this be implemented?):

Since T280226 no longer allows the user to continue purge pages after blocked, we may be able to do this in parts of the block.