This is an action-related message of the AbuseFilter extension.
abusefilter-action-*messages are shown on the main Special:AbuseFilter page in a column. They're short descriptions of what a filter does. They can be nouns or noun sentences, or verbs which have the AbuseFilter as subject.
abusefilter-edit-action-*messages are shown on the page of each filter, next to the checkbox through which the privileged user can "order" the filter to take that action, so it's usually best translated with an imperative or other verb sentence.
Users matching the filter will be blocked indefinitely, with a descriptive block summary indicating the rule that was triggered.
This is a verb.