Can I stop and start SEO services?

No! SEO is ongoing process. If you stop following SEO best practices once you reach the first position for your target keywords, then you likely won’t remain in that position long.

You don’t know how much your competitors are investing in SEO and Google is constantly updating their search algorithms. If you've put in the hard work to organically reach the first page of Google and want to continue to capitalize on the increase in traffic, then you need to continue following SEO best practices.

For more info, take a look at these SEO Best Practices