what to do if tenants do not acknowlege notices?

[Yiv L.]

I think the following scenarios can happen:
1. When a notice is served in person (ie 5-day notice pay rent or quit, termination notice, ...), what can we do if the tenant refuses to sign and acknowledge?
2. If the notice is sent by certified mail with signature, can tenant still argue in court that the letter he/she received has nothing to do with the subject on the case.

Please tell me my worries are unfounded.