As per the first reply - use the results filters to achieve the goal:
Search for all accounts
Click on the 'Spammers' filter in the results
Click the 'Prune Members found' button
Confirm the prune.
Screenshots...
After search, but before filter, clicking 'Prune Members found' results in:
After clicking the filter, clicking 'Prune Members found' results in:
Click Continue and the accounts will be marked for pruning and cleaned up via the background tasks.