Trend spamming probably wont be a problem early on, you'll have trusted users who'll build good content. At the start the people using your site will be early adopters, supportive friends and product evangelists who will be more interested in using your product rather than trying to spam with it
I'd advise that in early stages you concentrate on building up these users and observe their behaviour to see how they tag and ask them about the process. Spamming of any sort only real works if there's high volume and I'm deducing from the phrase 'in the process of developing' that you are not at this stage. Definitely don't annoy early users by putting in place a system that hinders their legitimate use of the site.
You can always directly remove any troublesome users at any stage.
The best thing to do long term is to allow community moderation to take place. Let the users vote stuff up and float that to the top of searches, that way any poorly tagged stuff is more likely to sink and be harder to find. Users will then be free to tag as they like but encouraged to tag nicely. Reward users who tag well and remove privileges from, or even delete those who tag poorly.
You could set a barrier to tag entry too where a user has to have a certain level of trusted status before they are allowed to add new tags that aren't in a predefined set. The same trusted status can allow the ability to re-tag anything that is poorly tagged. this means new users will have to behave to a good standard amongst their peers to use the site to it's full potential.
Crowd sourcing this sort of thing from your human users is a much better way than trying to mechanically restrict or suggest.
The best example of this is Stack Exchange.