Trojan in website

[ Moved to the Fixing WordPress sub-forum. ]

Wesite : www.performanceinsights.asia
I’m using WordPress 4.7.3 running AccessPress Parallax theme.
I’m facing issue with malware/trojan attacks my website. How do i remove them manually?

Symptom: can’t access website, divert to advert. try a few times only able to access.

Action done : Previously scan the web using free online tools and based on the reports saying some of the plugining is contaminated. Remove all the affected plugin.

Conclusion : The website still infected even after plugins are removed.

Question : how does the website getting attack? Is it hosting server security issue? Is it one of the plugins infected? Is it the theme itself is vulnerable?
How can i remove the malware/trojan without buying any tools?

I’m totally new to this kind of attack. Need help from all the gurus out there..