There's technically no rule against it but the amount time and effort it would take would be...astounding. Let's say you get 25 coins for the free adds you watch and you watch 5 ads per day. That's at least what I get when I get free coins although I may only get to watch 1-2 ads, or sometimes no ads.
To get 25,000 coins (which I think is the necessary number for the $25.00, correct me if I'm wrong on that.)
Let's say you got 125 coins per day. That makes it 875 coins per week, which then goes to around 3500 per month. It would still take around 7 months to reach that quota. And again, I'm being pretty generous with both the coins and the ads. I do know that some users will earn less coins than others, maybe due to not being on as long. I know that many people will not get as many ads.
Quite honestly, if you spent around a year clicking on ads and building up your tiny bucket of coins, then happy birthday. Here's $25 for your hard work of working with Tapas' ad program. Why don't you use it to buy more coins.