Should I list people who are in competition with me as reviewers to exclude for "conflict of interest"?

In my experience, when a conference asks authors to identify reviewers that are conflicted, they want you to identify reviewers who fall into certain categories of people who should not review your work because they have a relationship with you personally that could bias their review. For example: your advisor, your colleagues, your current collaborators, your family members, etc. They are not asking you to list reviewers who you consider to be your competition.

For example, the instructions for POPL 2017 say:

As an author, you should list PC and ERC members (and any others, since others may be asked for outside reviewers) which you believe have a conflict with you. While particular criteria for making this determination may vary, please apply the following guidelines, identifying a potential reviewer Bob as conflicted if

• Bob was your co-author or collaborator at some point within the last 2 years
• Bob is an advisor or advisee of yours
• Bob is a family member
• Bob has a non-trivial financial stake in your work (e.g., invested in your startup company)

Also please identify institutions with which you are affiliated; all employees or affiliates of these institutions will also be considered conflicted.

If a possible reviewer does not meet the above criteria, please do not identify him/her as conflicted. Doing so could be viewed as an attempt to prevent a qualified, but possibly skeptical reviewer from reviewing your paper. If you nevertheless believe that a reviewer who does not meet the above criteria is conflicted, you may identify the person and send a note to the PC Chair.

The answer is no.

Being in competition with you for acceptance is not a valid reason to exclude reviewers. That competition is implicit and you'd be excluding everyone then. Peer review assumes good faith on both authors and reviewers sides.

In your question you seem to imply that some reviewers are being dishonest, that is an entirely other issue.

You should read the guidelines carefully. Not every field/journal/conference will see things the same way. In particular it may depend heavily on whether you are asked for a reason for exclusion. Note that previous co-authors are easily spotted in bibliometric systems (including the journal's in-house system if you habitually publish there); competitors are harder to find.

According to AIP's ethics guidelines.

Privileged information or ideas obtained through peer review must be kept confidential and not used for competitive gain. Reviewers must disclose conflicts of interest resulting from direct competitive, collaborative, or other relationships with any of the authors, and avoid cases in which such conflicts preclude an objective evaluation.

Thus the onus is on the reviewers. However (from memory, if I'm wrong I'm thinking of another publisher) when you're asked for reviewers to avoid, you're also asked to provide a reason. By stating that you are competing with another group you can help the editor make a judgement call bearing in mind that in the editors' responsibilities section:

Situations that may lead to real or perceived conflicts of interest should be avoided.

The editor may think "perfect, someone who can review this really critically" and choose to ignore your suggestion (that's all it usually is) but then they have to be able to stand by this decision. It could affect how they choose the other reviewer(s). But you need to be specific and polite -- not "Prof X has it in for me" but "Prof X's group are working on very similar material and we feel it would be a conflict of interest if they were to see this work ahead of publication".