Can a person be punished for an ongoing sin after death, as a result of having previously entered a forbidden website, despite their repentance?

Ignore the whispers (waswas). As long as you have repented from the sin, Allah accepts your repentance, and you will not be held accountable for it. Even if we assume that the sin has an effect after repentance, it will not harm you. So, abandon the whispers and proceed in obedience to your Lord, keen on fulfilling the obligatory duties, avoiding the forbidden acts, and increasing in supererogatory acts of worship.