Does poking finger up anus weaken anus? If it does what are the remedies for a weak anus.

It can. Any forced stretching or other trauma, especially if repeated, can weaken anal sphincter tone. Many people engage in anal play, but any activities that are painful or are associated with weakened tone, especially resulting in unexpected/uncontrolled stool leakage, should be stopped. Over time without trauma, tone may return. If it does not, interventions including surgery may be required.
It can. Causing trauma to the anus can cause weakening, especially if there is a tear. Slight trauma can heal on its own without special intervention. A severe tear may need surgical intervention. Clenching exercises can help with strengthening.