This is an interesting question and I beleive the people at Unity08 are right in allowing candidates to stay in the draft process against their will for a certain amount of time. Politicians change their minds all the time.
If a politician is legally drafted for a nomination, he/she should only be allowed to withdraw his/her nomination after the politician gets the required amount of support to advance to the status of "Candidate" from "Prospective Candidate." If a reluctant politician can see the number of people declaring their support for him/her on their own behalf, then the politician might be encouraged to change his/her mind. Some mechanism must be in place for candidates and delegates to see how many delagates have signed up to support a nomination.
This sounds like it will simply add another layer to the bureaucracy. After the first round of voting, candidates and prospective candidates should be informed of their progress. If the prospective candidate does not wish to continue with the process, he/she should be allowed to bow out gracefully.