Hello Sir,
How can we prevent user from entering mobile number or website link in title and description of the form while posting an ad on the portal.

You cannot.
What you could do is to make keypress/keyuo jQuery function on title that does not allow special chars to be entered - like @, numbers 1-9, dot, ...

Ok Sir i have done that already on my portal but still there are users who still post links in description and title.

Then its not working and you must add PHP validation (use posted_item or pre_post_item hooks)

Sir it will be very helpful if you can provide code for it.

I do not have it, you need to build it.
Check these for hooks:
https://docs.osclasspoint.com/hooks

Sir how much it will cost to build code for this.