Time limits
Many events of ten miles or longer announce when the course will be closed on race day. At that time, aid stations will be dismantled, course marshals dismissed, and the finish line area broken down.
Since many events attract walkers and underprepared runners, a stated time limit is meant to dissuade those who can't finish in time.
The Laguna Hills Half-Marathon, in Laguna Hills CA, states how much time registrants will have to complete the course, how that translates to a miles per minute pace, and at what time the course and finish line chutes will be closed.
Most runners can complete any race at a pace of at least 15 minutes per mile. Anybody slower than that is mixing walking and running. A race has no obligation to keep the course open for everyone.
If you announce time limits - stick to them. If you don't enforce your own rules, participants will not follow them in the future.