First of all you can't jail the PAP because it is a political party and not a person. You can only jail a person. Putting a political party in jail is meaningless, because who would actually be the one doing the time?
Secondly, if you are talking about imprisoning certain individual PAP members, who do you have in mind and what charges would you bring against them?
Thirdly, do you actually think that our judges would ever make a decision against them? Consider the case of the 1997 Cheng San counting centres, where then-AG Chan Sek Keong refused to prosecute Goh Chok Tong and Lee Hsien Loong for an offence under the Parliamentary Elections Act. The rules clearly state that no candidates are allowed to loiter within a 200m radius of any polling station. Chan declared that since Goh and Lee were actually INSIDE the polling station, they were not within a 200m radius, and therefore they did not commit any offence.
There will always be technical loopholes that can be used to explain how legal or judicial decisions are made.
Chan Sek Keong later went on to become Chief Justice.