曾因違反居留條例而被判坐監的非法入境者，事隔五年，會不會被拒入境？

I will assume when you say "違反居留條例", you mean 違反入境條例.

Unless you've been deported or removed from HK (遞解離境/遣送離境), you will be allowed to enter HK as long as you can satisfy the immigration officer that you can support yourself in HK and have intention to leave after your visitor status expire.