A) Punishments decay over games played and time.
If you play a regular amount, it takes between three and six months to decay one punishment tier.
If you don't play at all, the punishment tier will stay exactly the same. There's no way to prove that you've changed if you don't show it in games.
If you spam 50 games a day, it will decrease slightly faster than playing a regular amount, but you can't blitz your way out of the punishment system, it will always take time.
B) The punishment tiers are:
No Punishment
10 Game Chat Restriction
25 Game Chat Restriction
14 Day Ban
Permanent Ban
Some things, like urging self-harm or hate speech will give escalated penalties, normally skipping two tiers of punishment. Sometimes, if you've had a 10 Game Chat Restriction it will still give you a 14 Day Ban instead of a Permanent Ban.
Other forms of extreme toxicity can also skip punishment tiers. It is even possible to skip strait to a Permanent Ban from No Punishment.
C) There are other things to be punished for aside from chat.
The first time the Feederbuster catches someone feeding it's a 14 Day Ban. The second is a Permanent Ban.
The first time gameplay trolling is detected it's a 14 Day Ban. The second is a Permanent Ban.
The first time caught elo boosting is a 14 Day Ban. The second is a Permanent Ban.
The first time caught account sharing can be either a 14 Day Ban or a Permanent Ban.
The first time caught scripting is a Permanent Ban
D) Your account can be locked for non-punishment reasons.
If you bought Riot Points, but Riot did not receive the money your account will be locked until Riot is paid.
If Riot detects suspicious activity on your account they will lock it until you prove that you are the original owner.